Abstract:
A focusing and leveling apparatus, including: an illumination light source ( 201 ) for emanating a light beam; an illumination lens group ( 202 ), a projecting unit, an object ( 209 ) being measured, a detecting unit, a refractive unit, a relay lens group and a photoelectric detector ( 220 ), disposed sequentially in this order in a transmission path of the light beam. The projecting unit includes projection slits ( 203 ) defining a plurality of non-linearly arranged marks, such that after passing sequentially through the illumination lens group ( 202 ), the projecting unit, the object ( 209 ) being measured and the detecting unit, the light beam emanated by the illumination light source ( 201 ) becomes a plurality of non-linearly arranged sub-beams corresponding to the plurality of marks. The refractive unit is so configured that after passing through the refractive unit, the plurality of sub-beams form a plurality of linearly arranged light spots.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to photolithography and, more particularly, to an apparatus having a refractive unit for automatic focusing and leveling in a photolithography tool. 
       BACKGROUND 
       [0002]    Current projection photolithography systems typically include an automatic focusing and leveling sub-system for accurately measuring a level and a tilt of the surface of an object.  FIG. 1  is a schematic illustration of such a focusing and leveling system used in a conventional projection photolithography tool. As shown in  FIG. 1 , the focusing and leveling system includes a projection objective  19  and an optical measuring path extending on both sides of an optical axis of the projection objective  19 . Disposed in the optical measuring path are an illuminating unit, a projecting unit, a detecting unit and a relay lens group arranged in this order. The illuminating unit is composed of an illumination light source  1 , an illumination lens group  2  and fiber optics (not shown). Light emanated from the light source  1  is condensed by the illumination lens group  2  and directed through the fiber optics onto the projecting unit, thereby providing illuminating light to the whole measuring system. The projecting unit is composed of projection slits  3 , an upstream projection lens group  4 , a reflector group  5  and a downstream projection lens group  6 . Light exiting the illuminating unit passes sequentially through the projection slits  3 , the upstream projection lens group  4 , the reflector group  5  and the downstream projection lens group  6 , and then forms light spots to be measured in an exposure area on the surface of the object  7  being measured. The detecting unit is composed of an upstream detection lens group  8 , a reflector group  9  and a downstream detection lens group  10 . The relaying unit is disposed downstream relative to the detecting unit and is made up of a relay reflector  11 , a relay lens group  12 , a detector  13 , a focusing controller  14  and an arithmetic unit  15 . When travelling in the relaying unit, light forming the spots is received by the detector  13  and converted thereby to a light intensity signal indicative of the position and tilt of the surface of the object being measured. 
         [0003]    As disclosed in one of its embodiments, the U.S. Pat. No. 5,414,515 provides a device with a single optical path and multiple light spots, in which, in order to achieve a measurement field of view having a required size, a plurality of charge-coupled devices (CCDs) is used. This design is disadvantageous in that the use of multiple CCDs, as wells as, corresponding image capture cards and cables, leads to high cost and increased footprint constraints on the whole photolithography tool in which the device is employed. 
         [0004]    The Chinese patent CN101710291 discloses a solar energy device having a lens for refracting parallel light beams. This device differs from the subject matter of the present invention in terms of its application. CN101710291 focuses on the convergence of light beam energy and has no requirement on image quality. In contrast, the present invention is applied to imaging position adjustment and has requirements on resolution, distortion, focal depth and other image quality metrics. Therefore, there are essential differences between CN101710291 and the present invention. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an objective of the present invention to provide a novel structure for focusing and leveling sub-systems in projection photolithography tools, in which a refractive unit is introduced in an optical path, thereby satisfying the system requirements for a large field of view and multipoint measurement with a single optical path and a single CCD linear array and resulting in overall reductions in cost and footprint. 
         [0006]    A focusing and leveling apparatus according to the present invention includes: an illumination light source for emanating a light beam; and an illumination lens group, a projecting unit, an object being measured, a detecting unit, a refractive unit, a relay lens group and a photoelectric detector, disposed sequentially in this order in a transmission path of the light beam. The photoelectric detector is configured to convert the light beam that has exited the relay lens group and carries information about a level and a tilt of the surface of the object being measured to an electrical signal and pass the electrical signal on to an arithmetic unit and a focusing and leveling controller. The arithmetic unit is configured to produce a control signal for the focusing and leveling controller based on the received electrical signal such that the focusing and leveling controller controls a wafer stage that supports the object being measured so as to adjust the level and tilt of the surface of the object being measured. The projecting unit includes projection slits defining a plurality of non-linearly arranged marks, such that after passing sequentially through the illumination lens group, the projecting unit, the object being measured and the detecting unit, the light beam emanated by the illumination light source becomes a plurality of non-linearly arranged sub-beams corresponding to the plurality of marks. The refractive unit is so configured that after passing through the refractive unit, the plurality of sub-beams form a plurality of linearly arranged light spots. 
         [0007]    Preferably, the projection slits define at least three non-linearly arranged marks. 
         [0008]    Preferably, the projecting unit further comprises a double-prism group having two prisms sandwiching the projection slits therebetween. 
         [0009]    Preferably, the refractive unit comprises a reflector group, a lens group and a prism group. 
         [0010]    Preferably, the reflector group consists of n−1 pairs of reflectors, the lens group consists of n lenses, and the prism group consists of n−1 prisms, where n corresponds to the number of the non-linearly arranged marks defined by the projection slits. 
         [0011]    Preferably, the refractive unit further includes a parallel-sided plate, wherein a middle one of the plurality of sub-beams incident on the refractive unit passes through a corresponding one of the lenses and is then incident on the parallel-sided plate, with the remaining ones of the plurality of sub-beams traveling sequentially through corresponding pairs of the reflectors, corresponding ones of the lenses and prisms and then being incident, together with the middle sub-beam, on the relay lens group. 
         [0012]    Preferably, the photoelectric detector is a single CCD linear array. 
         [0013]    Preferably, the refractive unit includes two wedges disposed with their sloped faces facing each other. 
         [0014]    Preferably, a first one of the two wedges defines first through-holes in its middle and lower parts at locations corresponding to incidence of the middle and lower ones of the plurality of sub-beams on the first wedge of the refractive unit, and a second one of the two wedges defines second through-holes in its middle and upper parts at locations corresponding to incidence of the middle and upper ones of the plurality of sub-beams on the second wedge of the refractive unit. 
         [0015]    Preferably, the refractive unit further includes a parallel-sided plate so disposed that the middle one of the plurality of sub-beams passes through the first and second through-holes defined in the respective middle parts of the first and second wedges and is then incident on the parallel-sided plate. 
         [0016]    Differing from the two schemes commonly adopted in the prior art, i.e., a first scheme having multiple optical paths and producing multiple light spots to be measured, in which a plurality of CCD linear arrays are used to meet the requirement for a large measurement field of view, and a second scheme having a single optical path and producing multiple light spots to be measured, in which a single CCD planar array is used to achieve a large measurement field of view, the present invention provides a novel structure for focusing and leveling sub-systems in projection photolithography tools, in which a refractive unit is introduced in an optical path, thereby satisfying the system requirements for a large field of view and multipoint measurement with a single optical path and a single CCD linear array and resulting in overall reductions in cost and footprint. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The advantages and spirit of the invention will be better understood from the following detailed description when considered with reference to the accompanying drawings, in which: 
           [0018]      FIG. 1  is a schematic illustration of a focusing and leveling system of a projection photolithography tool; 
           [0019]      FIG. 2  schematically illustrates a focusing and leveling system having a refractive unit according to the present invention; 
           [0020]      FIG. 3  schematically shows a focusing and leveling apparatus having a refractive unit with a reflector group according to the present invention; 
           [0021]      FIG. 4  is a partial view of the refractive unit with a reflector group according to the present invention; 
           [0022]      FIG. 5A  is a schematic illustrating projection slits in accordance with an embodiment of the present invention; 
           [0023]      FIG. 5B  shows locations on an image plane where light spots are formed before a light beam passes through the refractive unit in accordance with an embodiment of the present invention; 
           [0024]      FIG. 5C  shows locations on an image plane where light spots are formed after a light beam has passed through the refractive unit in accordance with an embodiment of the present invention; 
           [0025]      FIG. 6A  is a schematic illustrating projection slits in accordance with another embodiment of the present invention; 
           [0026]      FIG. 6B  shows locations on an image plane where light spots are formed before a light beam passes through the refractive unit in accordance with another embodiment of the present invention; 
           [0027]      FIG. 6C  shows locations on an image plane where light spots are formed after a light beam has passed through the refractive unit in accordance with another embodiment of the present invention; 
           [0028]      FIG. 7  shows locations on a CCD linear array where light spots are formed by the light beams of  FIGS. 5C and 6C ; 
           [0029]      FIG. 8  is a schematic illustrating a focusing and leveling apparatus having a refractive unit with a wedge group according to the present invention; and 
           [0030]      FIG. 9  is a partial view of the refractive unit with a wedge group according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Some specific Embodiments of the present invention will be described below with reference to the accompanying drawings. 
       Embodiment 1 
       [0032]      FIG. 2  schematically shows a focusing and leveling apparatus having a refractive unit for use in a projection photolithography tool according to the present invention. As illustrated, the apparatus comprises an illumination light source  101 , an illumination lens group  102 , projection slits  103 , a double-prism group  104 , an upstream projection lens group  105 , a projection diaphragm  106 , a downstream projection lens group  107 , a projection reflector  108 , an object  109  being measured, an upstream detection reflector  110 , an upstream detection lens group  111 , a detection diaphragm  112 , a downstream detection lens group  113 , a downstream detection reflector  114 , a refractive unit  115 , a relay lens group  116 , a CCD linear array  117 , a focusing controller  118 , and an arithmetic unit  119 . A light beam from the illumination light source  101  passes through the illumination lens group  102  and is then incident on the projection slits  103  which is sandwiched between the two prisms of the double-prism group  104 . After exiting the double-prism group  104 , sub-beams are generated and travel through the upstream projection lens group  105 , the projection diaphragm  106  and the downstream projection lens group  107 , and are then reflected by the projection reflector  108  onto the object  109 . The object  109  then reflects the sub-beams onto the upstream detection reflector  110 , which, in turn, reflects the sub-beams into the upstream detection lens group  111 . After traveling through the upstream detection lens group  111 , the sub-beams further propagate through the detection diaphragm  112  and the downstream detection lens group  113 , and are then reflected by the downstream detection reflector  114  into the refractive unit  115 . Upon exiting the refractive unit  115 , the sub-beams form light spots distributed in a single linear field of view, and then further transmit through the relay lens group  116  and are received by the CCD linear array  117 , where the light is subjected to photoelectric conversion and thereby results in a signal indicative of information about a level and a tilt of the surface of the object being measured. The signal is then passed by the focusing controller  118  on to the arithmetic unit  119 , where the signal is processed to perform control over a wafer stage  120  such that the object  109  is adjusted in its surface level and tilt as desired. 
         [0033]      FIG. 3  shows a focusing and leveling apparatus having a refractive unit for use in a focusing and leveling sub-system of a projection photolithography tool, constructed in accordance with Embodiment 1 of the present invention. The apparatus is used for accurate and efficient measurement of the position of a wafer surface. As illustrated, the apparatus comprises an illumination light source  201 , an illumination lens group  202 , projection slits  203 , a double-prism group  204 , an upstream projection lens group  205 , a projection diaphragm  206 , a downstream projection lens group  207 , a projection reflector  208 , an object  209  being measured, an upstream detection reflector  210 , an upstream detection lens group  211 , a detection diaphragm  212 , a downstream detection lens group  213 , a downstream detection reflector  214 , a reflector group  215  of the refractive unit, a lens group  216  of the refractive unit, a parallel-sided plate  217 , a prism group  218  of the refractive unit, a relay lens group  219 , a CCD linear array  220 , a focusing controller  221  and an arithmetic unit  222 . A light beam from the illumination light source  201  passes through the illumination lens group  202  and is then incident on the projection slits  203  which is sandwiched between the two prisms of the double-prism group  204 . After exiting the double-prism group  204 , sub-beams are generated and travel through the upstream projection lens group  205 , the projection diaphragm  206  and the downstream projection lens group  207 , and are then reflected by the projection reflector  208  onto the object  209 . The object  209  then reflects the sub-beams onto the upstream detection reflector  210 , which, in turn, reflects the sub-beams into the upstream detection lens group  211 . After traveling through the upstream detection lens group  211 , the sub-beams further propagate through the detection diaphragm  212  and the downstream detection lens group  213 , and are then reflected by the downstream detection reflector  214  onto the reflector group  215  (i.e., reflectors  215   a - 215   h ). After leaving the reflector group  215 , the sub-beams travel through the lens group  216  (i.e., respectively through lens  216   a - 216   e ), and then a middle one of the sub-beams passes through the parallel-sided plate  217 , with all the rest, peripheral ones of the sub-beams passing through the prism group  218  (i.e., respectively through prisms  218   a - 218   d ), wherein mark B corresponds to  215   a ,  215   b ,  216   a  and  218   a , mark A to  215   c ,  215   d ,  216   b  and  218   b , mark D to  215   e ,  215   f ,  216   c  and  218   c , and mark E to  215   g ,  215   h ,  216   d  and  218   d . Thereafter, all the sub-beams, including the middle one, transmit through the relay lens group  219  and are received by the CCD linear array  220 , where they are subjected to photoelectric conversion and result in information about a level and a tilt of the surface of the object being measured. The information is then transferred by the focusing controller  221  to the arithmetic unit  222 , in order to perform control over a wafer stage  223  such that the object  209  is adjusted in its surface level and tilt as desired. While the sub-beams have been illustrated as being incident on the refractive unit in a horizontal orientation in this embodiment, the present invention is not limited to such horizontal incidence in practical applications. The reflector group  215  functions to separate the respective sub-beams, which form respective images after passing through the lens group  216 , and are then refracted by the prism group  218  onto the relay lens group  219 . Finally, the images formed by the sub-beams appear in a linear arrangement on the CCD linear array  220 .  FIG. 4  is a partial view of the refractive unit having the reflectors. As shown in the figure, the refractive unit includes the reflector group  215 , the lens group  216 , the parallel-sided plate  217  and the prism group  218 . After the sub-beams are reflected by the reflector group  215 , they are incident on and travel through the lens group  216 . Upon exiting the lens group  216 , the middle one of the sub-beams transmits through the parallel-sided plate  217 , and the remaining four peripheral sub-beams all pass through the prism group  218 . Thereafter, the peripheral sub-beams, together with the middle sub-beam, pass through the relay lens group  219  and are received by the CCD linear array  220 . 
         [0034]      FIGS. 5A and 6A  show two applicable layouts of the projection slits. In each case, the slits are disposed between the two prisms of the double-prism group  204  in a tilted configuration, and the marks A, B, C, D and E of the projection slits are arranged at the locations indicated in the respective figure.  FIGS. 5B and 6B  show spatially distributed light spots A′, B′, C′, D′ and E′ formed on an image plane before the sub-beams pass through the refractive unit in cases of the respective layouts of the projection slits being used.  FIGS. 5C and 6C  show linearly distributed light spots A″, B″, C″, D″ and E″ formed on an image plane after the sub-beams have passed through the refractive unit in cases of the respective layouts of the projection slits being used. In each case, the sub-beams finally travel through the relay lens group  219  and are received by the CCD linear array  220 , thereby converting the planar distribution of the light spots in the field of view to a desired linear distribution. 
         [0035]      FIG. 7  shows light spots formed by the sub-beams on the CCD linear array in both of the cases shown in  FIGS. 5C and 6C , wherein the light spots appear as linearly arranged patterns A′″, B′″, C′″, D′″ and E′″. 
       Embodiment 2 
       [0036]      FIG. 8  is a schematic illustration of a focusing and leveling apparatus having a refractive unit with a wedge group for use in a focusing and leveling sub-system of a projection photolithography tool, constructed in accordance with Embodiment 2 of the present invention. The apparatus is used for accurate and efficient measurement of the position of a wafer surface. As illustrated, the apparatus comprises an illumination light source  301 , an illumination lens group  302 , projection slits  303 , a double-prism group  304 , an upstream projection lens group  305 , a projection diaphragm  306 , a downstream projection lens group  307 , a projection reflector  308 , an object  309  being measured, an upstream detection reflector  310 , an upstream detection lens group  311 , a detection diaphragm  312 , a downstream detection lens group  313 , a downstream detection reflector  314 , a wedge group  315  of the refractive unit, a parallel-sided plate  316 , a relay lens group  317 , a CCD linear array  318 , a focusing controller  319  and an arithmetic unit  320 . A light beam from the illumination light source  301  passes through the illumination lens group  302  and is then incident on the projection slits  303  which is sandwiched between the two prisms of the double-prism group  304 . After exiting the double-prism group  304 , sub-beams are generated and travel through the upstream projection lens group  305 , the projection diaphragm  306  and the downstream projection lens group  307 , and are reflected by the projection reflector  308  onto the object  309 . The object  309  subsequently reflects the sub-beams onto the upstream detection reflector  310 , which, in turn, reflects the sub-beams into the upstream detection lens group  311 . After traveling through the upstream detection lens group  311 , the sub-beams further propagate through the detection diaphragm  312  and the downstream detection lens group  313 , and are then reflected by the downstream detection reflector  314  into the wedge group  315 . After exiting the wedge group  315 , a middle one of the sub-beams passes through the parallel-sided plate  316 , and then, together with the rest, peripheral ones of the sub-beams, transmits through the relay lens group  317  and is received by the CCD linear array  318 , where the sub-beams are subjected to photoelectric conversion and result in information about a level and a tilt of the surface of the object being measured. The information is then transferred by the focusing controller  319  to the arithmetic unit  320 , in order to perform control over a wafer stage  321  such that the object  309  is adjusted in its surface level and tilt as desired. 
         [0037]      FIG. 9  is a schematic illustration of the refractive unit having the wedge group. As shown in the figure, the refractive unit includes the parallel-sided plate  316  and the wedge group  315 . The wedge group  315  is composed of two wedges disposed with their sharp corners opposite to each other. After being reflected by the downstream detection reflector  314 , the sub-beams are imaged on a single image plane by the two wedges  315 . In the two wedges, the wedge  315   a  is disposed with its sharp corner oriented in the same direction as the orientation of the marks B and D (i.e., upward), and the sharp corner of the wedge  315   b  is oriented in the same direction as the orientation of the marks A and E (i.e., downward). The wedge group deflects the sub-beams by an angle given as: 6=a (n−1), where, n is the index of refraction of the wedges, and a is the angle of the sharp corners thereof which is less than 5°. The sub-beams are incident on a face of the upstream wedge  315   a  at an angle that is desired to be as near to 90° as possible, and is finally imaged on the CCD linear array  318  by the relay lens group  317 . The wedge  315   a  defines two holes at locations corresponding to the incidence of the lower two of the aforementioned sub-beams, while the wedge  315   b  defines two holes at locations corresponding to the incidence of the upper two of the sub-beams that have been deflected by the wedge  315   a . In addition, each of the wedges  315   a  and  315   b  further defines a hole corresponding to the incidence of a middle one of the sub-beams. All of these holes are through-holes without deflecting the sub-beams during their passage therethrough. Further, the parallel-sided plate  316  is disposed to compensate for an optical path difference of the middle sub-beams, such that the planar distribution of the light spots is converted to a desired distribution after the sub-beams have passed through the wedge group  315 , and the sub-beams then transmit through the relay lens group  317  and are finally received by the CCD linear array  320 . 
         [0038]    Disclosed herein are merely several preferred embodiments of the present invention, which are presented only to illustrate rather than limit the invention in any way. Any other technical schemes resulting from logical analysis, inference or limited experimentation by those skilled in the art in light of this inventive concept is considered to fall within the scope of the present invention.