Abstract:
A lithography projection objective ( 30 ) for focusing and imaging a pattern of a reticle onto a wafer including, from the reticle and along an optical axis: a first lens group G 31  having a positive refractive power; a second lens group G 32  having a positive refractive power; a third lens group G 33  having a positive refractive power; and a fourth lens group G 34  having a positive refractive power. These four lens groups form a 2× magnification design which has a partial field of view of not smaller than 100 mm; a wavelength band of I-line±5 nm can ensure a sufficient exposure light intensity. Moreover, the present invention also achieves, with a relatively simple structure, the demanded millimeter-level resolution as well as the correction of distortions, field curvatures, astigmatisms and chromatic aberrations in a large field.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates in general to semiconductor processing technologies, and more particularly, to a large field projection objective for use in projection optical systems of lithography tools. 
       BACKGROUND 
       [0002]    Currently, in the field of semiconductor processing, there are increasing demands for high-throughput projection optical systems with millimeter-level resolution. In order to obtain a high throughput, stepper-type lithography tools generally adopt a large exposure field. Moreover, in order to be adapted for reticle size, some of them employ optical systems with a magnification of 1.25× or 1.6×. 
         [0003]    Japanese patent publication No.2000199850 discloses a 1.6× projection objective for lithography using exposure light with a wavelength of G-line or H-line and having a 117.6 mm field of view and a numerical aperture (NA) of 0.1 on a wafer surface. The projection objective is a multi-lens system consisting of 38 lenses and including an aspheric surface. 
         [0004]    Moreover, Japanese patent publication No. 2006267383 discloses a 1.25× projection objective for lithography using I-line with a wavelength band of ±3 nm as the exposure light and has a partial field of 93.5 mm 
         [0005]    Furthermore, Japanese patent publication No. 2007079015 discloses another 1.25× projection objective also using I-line with a wavelength band of +1.5 nm as the exposure light and has a partial field of 93.5 mm. 
         [0006]    Indicated as above, such large exposure field design is dominant in the field of liquid crystal display (LCD) lithography tool, and meanwhile, in order to be adapted for reticle size, many optical systems adopt projection objectives with a magnification of higher than 1×, and even close to 2×. According to these conventional technologies in combination with consideration of indeed demands, there is a need to develop a projection objective with a magnification of 2×. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is directed to the provision of a large field projection objective which is able to correct multiple types of aberrations, in particular, distortions, field curvatures, astigmatisms, axial chromatic aberrations and magnification chromatic aberrations, and obtain telecentricity both on the object and image sides. 
         [0008]    In one embodiment, a projection objective for lithography which focuses and images a pattern of a reticle onto a wafer successively comprises, from the reticle and along an optical axis: a first lens group G 31  having a positive refractive power; a second lens group G 32  having a positive refractive power; a third lens group G 33  having a positive refractive power; and a fourth lens group G 34  having a positive refractive power, wherein following formulae are satisfied: 
         [0000]      1.8 &lt;|fG 32/ fG 31|&lt;5.4 
         [0000]      0.57&lt;| fG 33/ fG 34|&lt;0.97 
         [0000]      0.19&lt;| IfG 33/ fG 32|&lt;0.5 
         [0009]    where f G31  is a focal length of the first lens group G 31 , f G32  is a focal length of the second lens group G 32 , f G33  is a focal length of the third lens group G 33 , and f G34  is a focal length of the fourth lens group G 34 . 
         [0010]    Preferably, the first lens group G 31  comprises at least four lenses. Additionally, the second lens group G 32  comprises at least six lenses and includes at least two pairs of lenses each consisting of a positive lens and a negative lens adjacent to the positive lens. Moreover, the third lens group G 33  comprises at least four lenses and includes a sub-lens group G 33 - 1   n  having a positive refractive power, the sub-lens group G 33 - 1   n  including at least two adjacent lenses of the third lens group G 33  both of which have a positive refractive power. Furthermore, the fourth lens group G 34  comprises at least six lenses and includes a sub-lens group G 34 - 1   n  having a positive refractive power, the sub-lens group G 34 - 1   n  including at least three directly successively arranged lenses of the fourth lens group G 34  all of which have a positive refractive power. The following formulae are satisfied: 
         [0000]      1.03&lt;| f   el     —     max   /f   G31 |&lt;1.95 
         [0000]      0.34&lt;| f   G33-1n   /f   G33 |&lt;0.87 
         [0000]      0.21&lt;| f   G34-1n   /f   G34 |&lt;0.47 
         [0011]    where f el     —     max  is a focal length of a lens of the first lens group G 31  which has a greatest refractive power in the first lens group G 31 , f G33-1n  is a focal length of the sub-lens group G 33 - 1   n  of the third lens group G 33 , and f G34-1n  is a focal length of the sub-lens group G 34 - 1   n  of the fourth lens group G 34 . 
         [0012]    Preferably, the second lens group G 32  includes at least a positive lens and a negative lens directly adjacent to the positive lens and the following formula is satisfied: 
         [0000]      1.23&lt; V   G32-p   /V   G32-N &lt;1.85 
         [0013]    where V G32-p  is an Abbe number of the positive lens of the second lens group G 32 , and V G32-N  is an Abbe number of the negative lens of the second lens group G 32  that is directly adjacent to the positive lens. 
         [0014]    Preferably, the second lens group G 32  includes at least a positive lens and a negative lens directly adjacent to the positive lens, and the following formula is satisfied: 
         [0000]      b  1 . 59 &lt; V   G32-P   /V   G32-N &lt;2.65 
         [0015]    where V G32-P  is an Abbe number of the positive lens of the second lens group G 32 , and V G32-N  is an Abbe number of the negative lens of the second lens group G 32  that is directly adjacent to the positive lens. 
         [0016]    Preferably, the two adjacent positive lenses of the sub-lens group G 33 - 1   n  of the third lens group G 33  satisfy the following formula: 
         [0000]      0.75&lt; f   41   &lt;f   42 1 
         [0017]    where f 41  is a focal length of the lens disposed upstream in the direction from the reticle to the wafer, and f 42  is a focal length of the lens disposed downerstream in the direction from the reticle to the wafer. 
         [0018]    Preferably, the projection objective is made of at least two groups of high refractive index mateirals and at least two groups of low refractive index mateirals. 
         [0019]    Preferably, the high refractive index mateirals are mateirals having a refractive index of higher than 1.55 at I-line, including a first material group whose materials have refractive indices which are higher than 1.55 at I-line and Abbe numbers which are higher than 45 and a second material group whose materials have refractive indices which are higher than 1.55 at I-line and Abbe numbers which are higher than 50; the low refractive index mateirals are mateirals having a refractive index of lower than 1.55 at I-line, including a third material group whose materials have refractive indices which are lower than 1.55 at I-line and Abbe numbers which are lower than 55 and a fourth material group whose materials have a refractive indices which are lower than 1.55 at I-line and Abbe numbers which are higher than 60. 
         [0020]    Preferably, both of a first lens of the first lens group G 31  and a last lens of the fourth lens group G 34  are made of a material of the first material group. 
         [0021]    Preferably, each of the first, second, third and fourth lens groups includes at least one lens made of the a material of the first or second material group. 
         [0022]    Preferably, each of the first, second and fourth lens groups includes at least one lens made of a material of the first material group. 
         [0023]    Preferably, the third lens group includes at least one lens made of a material of the second material group. 
         [0024]    Preferably, the second lens group includes at least one pair of lenses each of which has a convace surface facing a convace surface of the other lens. Additionally, the third lens group includes at least one meniscus lens which has a concanve surface facing an image plane. Moreover, the fourth lens group includes at least one meniscus lens which has a concave surface facing an object plane. 
         [0025]    The present invention achieves, with a smaller number of lenses, a 2× projection objective for lithography which has a partial field of view of not smaller than 100 mm and is suited for I-line light with a wavelength band of +5 nm which can ensure a sufficient exposure light intensity. At the same time, the present invention also achieves, with a relatively simple structure, the demanded millimeter-level resolution as well as the correction of distortions, field curvatures, astigmatisms and chromatic aberrations in a large field. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    To provide a more complete understanding of the present invention and its advantages, reference is made to the following detailed description on example embodiments, taken in conjunction with the accompanying drawings, in which: 
           [0027]      FIG. 1  schematically illustrates the optical structure of a projection objective for lithography according to an embodiment of the present invention; 
           [0028]      FIG. 2  shows a curve representing the distortions in an embodiment of the present invention; 
           [0029]      FIG. 3  shows curves representing telecentricity on the object and image sides in an embodiment of the present invention; and 
           [0030]      FIG. 4  shows curves representing aberrations in an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Several exemplary embodiments of the present invention will be described below with reference to the accompanying drawings. 
         [0032]    Referring to  FIG. 1 , a projection objective  30  according to an embodiment of the present invention includes twenty lenses, and specifications of which are shown in the following Table 1. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Working Wavelength 
                 @365 nm ± 5 nm 
               
               
                   
                 Image-side Numerical Aperture 
                 0.1 
               
               
                   
                 (NA) 
               
               
                   
                 Magnification 
                 −2 
               
               
                   
                 Image-side Filed of View 
                  200 mm 
               
               
                   
                 (diameter) 
               
               
                   
                 Object-Image Distance 
                 1500 mm 
               
               
                   
                   
               
             
          
         
       
     
         [0033]    The projection objective  30  is composed of twenty lenses. The twenty lenses are all spherical lenses and can be divided into four lens groups G 31 , G 32 , G 33  and G 34  each having a positive refractive power. 
         [0034]    The first lens group G 31  consists of four lenses, refractive powers of which are negative, positive, positive and positive, respectively. 
         [0035]    The second lens group G 32  consists of six lenses, refractive powers of which are positive, negative, negative, positive, positive and negative, respectively. Moreover, the second lens group G 32  includes at least two pairs of lenses each consisting of a positive lens and a negative lens adjacent to the positive lens. Furthermore, the second lens group G 32  includes at least one pair of lenses each of which has a convace surface facing a convace surface of the other lens. 
         [0036]    The third lens group G 33  consists of four lenses, refractive powers of which are positive, positive, negative and negative, respectively. Additionally, the third lens group G 33  includes a sub-lens group G 33 - 1   n  having a positive refractive power, the sub-lens group G 33 - 1   n  including at least two adjacent lenses of the third lens group G 33  both of which have a positive refractive power. Moreover, the third lens group G 33  includes at least one meniscus lens which has a concanve surface facing an image plane. 
         [0037]    The fourth lens group G 34  consists of six lenses, refractive powers of which are negative, positive, positive, positive, positive and negative, respectively. Moreover, the fourth lens group G 34  includes a sub-lens group G 34 - 1   n  having a positive refractive power, the sub-lens group G 34 - 1   n  including at least three directly successively arranged lenses of the fourth lens group G 34  all of which have a positive refractive power. In addition, the fourth lens group G 34  includes at least one meniscus lens which has a concanve surface facing an object plane. 
         [0038]    The projection objective  30  is made of at least two groups of high refractive index mateirals and at least two groups of low refractive index mateirals, wherein the high refractive index mateirals may be those having a refractive index of higher than 1.55 for I-line light, including a first material group whose materials have refractive indices which are higher than 1.55 at I-line and Abbe numbers which are higher than 45 and a second material group whose materials have refractive indices which are higher than 1.55 at I-line and Abbe numbers which are higher than 50, while the low refractive index mateirals may be those having a refractive index of lower than 1.55 at I-line, including a third material group whose materials have refractive indices which are lower than 1.55 at I-line and Abbe numbers which are lower than 55 and a fourth material group whose materials have refractive indices which are lower than 1.55 at I-line and Abbe numbers which are higher than 60. 
         [0039]    In a preferred embodiment, each of the first, second, third and fourth lens groups includes at least one lens made of a material of the first or second material group. Each of the first, second and fourth lens groups includes at least one lens made of the a material of first material group. The third lens group includes at least one lens made of a material of the second material group. Furthermore, both of a first lens of the first lens group G 31  and a last lens of the fourth lens group G 34  are preferably made of a material of the first material group. 
         [0040]    The first lens group G 31  consists of four lenses  31 ,  32 ,  33  and  34 , among which: the lens  31  is a biconcave negative lens; the lens  32  is a positive meniscus lens having a concave surface facing a reticle surface R; and both the lenses  33  and  34  are positive lenses. Moreover, the lenses  31 ,  32  and  34  are each made of a material of the first or third material group while the lens  33  is made of a material of the second or fourth material group. 
         [0041]    The second lens group G 32  consists of six lenses  35 ,  36 ,  37 ,  38 ,  39  and  40 , among which: the lens  35  is a biconvex positive lens; the lenses  36  and  37  are negative lenses, and the lens  36  has a concave surface  362  facing a concave surface  371  of the lens  37 ; the lenses  38  and  39  are positive lenses; and the lens  40  is a negative lens. Additionally, lenses  35 ,  36 ,  38  and  39  are each made of a material of the second or fourth material group while lenses  37  and  40  are each made of a material of the first or third material group. 
         [0042]    The third lens group G 33  consists of four lenses  41 ,  42 ,  43  and  44 , among which: both the lenses  41  and  42  have a positive refractive power; both the lenses  43  and  44  have a negative refractive power; and the lens  43  is a meniscus lens having a concave surface  432  facing a wafer surface. Moreover, all the lenses  41 ,  42 ,  43  and  34  are each made of a material of the second or fourth material group. 
         [0043]    The fourth lens group G 34  consists of six lenses  45 ,  46 ,  47 ,  48 ,  49  and  50 , refractive powers of which are negative, positive, positive, positive, positive and negative, respectively. The lens  45  has a flat rear surface and a concave surface facing the reticle surface. In addition, the lenses  45  and  47  are each made of a material of the second or fourth material group while the lenses  46 ,  48 ,  49  and  50  are each made of a material of the first or third material group. 
         [0044]    Relationship among parameters of the lens groups G 31 , G 32 , G 33  and G 34  and their sub-lens groups can be expressed by the following formulae which further determines basic conditions for optimizing the imaging quality of the projection objective, 
         [0000]      1.8&lt;| f   G32   /f   G31 |&lt;5.4   (1)
 
         [0000]      0.57&lt;| f   G33   /f   G34 |&lt;0. 97   (2)
 
         [0000]      0.19&lt;| f   G33   /f   G32 |&lt;0.5   (3)
 
         [0000]      1.03&lt;| f   el     —     max   f   G31 |&lt;1.95   (4)
 
         [0000]      0.34&lt;| f   G33-1n   /f   G33 |&lt;0.87   (5)
 
         [0000]      0.21 &lt;|f   G34-1n   /f   G34 |&lt;0.47   (6)
 
         [0000]      0.75 &lt;f   41   /f   42 &lt;1   (7)
 
         [0000]      1.23&lt; V   G32-P   /V   G32-N &lt;1.85   (8)
 
         [0000]        1.59&lt;V   G32-P   /V   G32-N &lt;2.65   (9)
 
         [0045]    where: f G31  is a focal length of the first lens group G 31 ; f G32  is a focal length of the second lens group G 32 ; f G33  is a focal length of the third lens group G 33 ; f G34  is a focal length of the fourth lens group G 34 ; f el     —     max  is a focal length of a lens of the first lens group G 31  which has a greatest refractive power in the first lens group G 31 ; f G33-1n  is a focal length of the sub-lens group G 33 - 1   n  of the third lens group G 33 ; f G34-1n  is a focal length of the sub-lens group G 34 - 1   n  of the fourth lens group G 34 ; f 41  is a focal length of the lens  41  that is disposed upstream in the direction from the reticle to the wafer of the two adjacent lenses  41  and  42  of the sub-lens group G 33 - 1   n  of the third lens group G 33 , while f 42  is a focal length of the other lens  42  that is disposed downstream in the direction; and V G32-P  and V G32-N  are Abbe numbers of a positive lens of the second lens group G 32  and a negative lens of the second lens group G 32  that is adjacent to the positive lens, respectively. 
         [0046]    As described above, in this embodiment, focal lengths of the positive lenses  41  and  42  of the sub-lens group G 33 - 1   n  of the third lens group G 33  are indicated as f 41  and f 42  according to a relationship between their positions in the direction from the reticle to the wafer and satisfy the formula f 41 &lt;f 42 . These two positive lenses are used to gradually compress the light eminated from the second lens group and thus improve the correction of field curvatures. 
         [0047]    Moreover, the foregoing formulae (1) to (9) have defined structural relations of the lens groups G 31 , G 32 , G 33 , G 34  and their sub-lens groups in correcting aberrations. 
         [0048]    Specific design values of the projection objective in this embodiment are given in Table 2, in which, a positive radius of a surface indicates that a center of curvature of the surface is on its right side, and similarly, a negative radius of a surface indicates that a center of curvature of the surface is on its left side; a thickness of an optical member or a spacing between two optical members is defined as a distance from a current surface to the next surface along the optical axis; and the unit of all dimensions is millimeter. 
         [0049]    In Table 2, there is further indicated: surface number “S#”, aperture stop (AS) “STOP”, and infinity “INF” in the column of Radius. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 S 
                   
                 Thicknesses and 
                   
                   
               
               
                 # 
                 Radii 
                 Spacings 
                 Materials 
                 Remarks 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 OBJ 
                 INF 
                 47.0004 
                   
                 Object-side 
               
               
                   
                   
                   
                   
                 Working Distance 
               
               
                  1 
                 −341.576 
                 34.29386 
                 PBL25Y 
                 L1 
               
               
                  2 
                 1185.168 
                 12.21087 
               
               
                  3 
                 −222.541 
                 45.3383 
                 PBM18Y 
                 L2 
               
               
                  4 
                 −161.39 
                 71.85194 
               
               
                  5 
                 INF 
                 47.00983 
                 SFSL5Y 
                 L3 
               
               
                  6 
                 −183.839 
                 1 
               
               
                  7 
                 844.4744 
                 26.49825 
                 PBL25Y 
                 L4 
               
               
                  8 
                 INF 
                 194.2484 
               
               
                  9 
                 414.9022 
                 26.25041 
                 SFSL5Y 
                 L5 
               
               
                 10 
                 −243.136 
                 6.168656 
               
               
                 11 
                 −176.468 
                 23 
                 BSM51Y 
                 L6 
               
               
                 12 
                 254.7796 
                 31.33048 
               
               
                 13 
                 −178.33 
                 44.40986 
                 PBM18Y 
                 L7 
               
               
                 14 
                 −232.626 
                 29.66318 
               
               
                 15 
                 403.048 
                 25.85819 
                 SFSL5Y 
                 L8 
               
               
                 16 
                 −230.792 
                 1 
               
               
                 17 
                 INF 
                 30.67998 
                   
                 (STOP) 
               
               
                 18 
                 313.9001 
                 25.88924 
                 SFSL5Y 
                 L9 
               
               
                 19 
                 −331.934 
                 6.301843 
               
               
                 20 
                 −270.11 
                 23 
                 PBM18Y 
                 L10 
               
               
                 21 
                 288.0486 
                 64.21258 
               
               
                 22 
                 329.4014 
                 27.44817 
                 SFSL5Y 
                 L11 
               
               
                 23 
                 −680.652 
                 2.530852 
               
               
                 24 
                 186.3488 
                 28.49718 
                 BSL7Y 
                 L12 
               
               
                 25 
                 3221.473 
                 1.000735 
               
               
                 26 
                 161.8902 
                 29.67068 
                 BSM51Y 
                 L13 
               
               
                 27 
                 105.8729 
                 64.02459 
               
               
                 28 
                 −238.698 
                 48.93309 
                 BSM51Y 
                 L14 
               
               
                 29 
                 389.3077 
                 38.0038 
               
               
                 30 
                 −117.901 
                 23 
                 BSM51Y 
                 L15 
               
               
                 31 
                 INF 
                 20.13205 
               
               
                 32 
                 −320.124 
                 46.94477 
                 PBM18Y 
                 L16 
               
               
                 33 
                 −226.249 
                 4.830725 
               
               
                 34 
                 317.7772 
                 49.397 
                 BSL7Y 
                 L17 
               
               
                 35 
                 5543.824 
                 32.52187 
               
               
                 36 
                 345.6422 
                 49.34704 
                 PBM18Y 
                 L18 
               
               
                 37 
                 −1234.86 
                 1 
               
               
                 38 
                 330.8498 
                 49.41093 
                 PBM18Y 
                 L19 
               
               
                 39 
                 INF 
                 27.72791 
               
               
                 40 
                 −434.844 
                 37.4797 
                 PBM18Y 
                 L20 
               
               
                 41 
                 −12935.3 
                 100.8894 
                   
                 Image-side 
               
               
                   
                   
                   
                   
                 Working Distance 
               
               
                 IMG 
                 INF 
                 0 
               
               
                   
               
             
          
         
       
     
         [0050]      FIG. 2  shows that the projection objective  30  of this embodiment has a good performance in distortion inhibition. 
         [0051]      FIG. 3  shows that telecentricity is corrected to about 3 mrad on the object side and to about 10 mrad on the image side of the projection objective  30 . 
         [0052]    Moreover, the aberration curves in  FIG. 4  indicate that the projection objective  30  has a good performance in image quality correction, and that a high quality of images at I-line±5 nm has been achieved. 
         [0053]    In a word, the projection objectives described in the description are merely several preferable embodiments of the invention which are provided solely for the purpose of describing but not limiting the invention in any way. Any technical solutions which are obtained by those skilled in the art through logical analysis, reasoning or limited experiment in light of the conception of the invention are within the scope as defined in the appended claims.