Abstract:
An aspherical lens ( 10, 10 ′) has an aspherical surface ( 101, 101 ′) and an opposite flat surface ( 102, 102 ′). A multiple-step grating is defined in the flat surface by a photolithographic method. The aspherical lens has a reduced chromatic aberration effect, and can thus provide a clear image. Furthermore, an effect of the aspherical lens having the grating is equivalent to that of a conventional aspherical lens unit. Thus, when the aspherical lens is used in an optical system, it can reduce a bulk and a weight of the optical system.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to optical components such as those used in consumer products; and more particularly to aspherical lenses.  
           [0003]    2. Description of the Related Art  
           [0004]    Aspherical lenses are widely used in aspherical lens systems of vidicons, cameras and so on. U.S. Pat. No. 5,861,999 discloses a conventional aspherical lens system. The aspherical lens system comprises a plurality of aspherical lens units. Each aspherical lens unit comprises two aspherical lenses with a clearance defined therebetween. A chromatic aberration of the aspherical lens unit can be adjusted by adjusting the clearance. U.S. Pat. Nos. 5,557,472 and 5,513,045 also disclose such conventional aspherical lens systems.  
           [0005]    Optical characteristics of each aspherical lens unit are determined in large part by the physical geometry of each of the aspherical lenses thereof. If either aspherical lens is imprecisely formed, this can result in an unacceptably high chromatic aberration effect. Furthermore, the aspherical lens system comprising a plurality of aspherical lens units is relatively bulky and heavy, and unsuited for miniaturized applications.  
           [0006]    Therefore, a new kind of aspherical lens that overcomes the above-described problems is desired.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    Accordingly, an object of the present invention is to provide an aspherical lens having an ameliorated chromatic aberration effect.  
           [0008]    Another object of the present invention is to provide an aspherical lens that can be used in an aspherical lens system so as to reduce a bulk and a weight of the aspherical lens system.  
           [0009]    To achieve the above-mentioned objects, the present invention provides an aspherical lens having an aspherical surface and an opposite flat surface with a multiple-step grating defined therein.  
           [0010]    The aspherical lens of the present invention can reduce a chromatic aberration effect thereof by adjusting phases thereof by using the grating. Thus, the aspherical lens can provide a clear image. Furthermore, an effect of the aspherical lens having the grating is equivalent to that of a conventional aspherical lens unit. Thus, when the aspherical lens having the grating is used in an aspherical lens system, it can reduce a bulk and a weight of the aspherical lens system.  
           [0011]    Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a schematic side elevation of an aspherical lens in accordance with the present invention, but prior to formation of a grating therein;  
         [0013]    [0013]FIG. 2 is a schematic side cross-sectional view of a first mask used in a first optical lithograph process for fabricating the aspherical lens of the present invention;  
         [0014]    [0014]FIG. 3 is similar to FIG. 1, but showing a first photoresist film formed on the aspherical lens;  
         [0015]    [0015]FIG. 4 is similar to FIG. 3, but showing the first mask placed on the first photoresist film and a first exposure process being performed;  
         [0016]    [0016]FIG. 5 is similar to FIG. 4, but showing the first photoresist film changed after a first developing process has been performed;  
         [0017]    [0017]FIG. 6 is similar to FIG. 5, but showing the aspherical lens changed after a first etching process has been performed;  
         [0018]    [0018]FIG. 7 is similar to FIG. 6, but showing the aspherical lens after residual portions of the first photoresist film have been removed;  
         [0019]    [0019]FIG. 8 is a schematic side cross-sectional view of a second mask used in a second optical lithography process for fabricating a first embodiment of the aspherical lens of the present invention;  
         [0020]    [0020]FIG. 9 is similar to FIG. 7, but showing a second photo resist film formed on the aspherical lens;  
         [0021]    [0021]FIG. 10 is similar to FIG. 9, but showing the second mask placed on the second photoresist film and a second exposure process being performed;  
         [0022]    [0022]FIG. 11 is similar to FIG. 10, but showing the second photoresist film changed after a second developing process has been performed;  
         [0023]    [0023]FIG. 12 is similar to FIG. 11, but showing the aspherical lens changed after a second etching process has been performed;  
         [0024]    [0024]FIG. 13 is similar to FIG. 12, but showing the completed aspherical lens in accordance with the first embodiment of the present invention after residual portions of the second photo resist film have been removed;  
         [0025]    [0025]FIG. 14 is a schematic side elevation of a third mask used in a second optical lithograph process for fabricating a second embodiment of the aspherical lens of the present invention;  
         [0026]    [0026]FIG. 15 is similar to FIG. 9, except that the photo resist film formed on the aspherical lens is defined as a third photo resist film;  
         [0027]    [0027]FIG. 16 is similar to FIG. 15, but showing the third mask placed on the third photoresist film and a second exposure process being performed;  
         [0028]    [0028]FIG. 17 is similar to FIG. 16, but showing the third photoresist film changed after a second developing process has been performed;  
         [0029]    [0029]FIG. 18 is similar to FIG. 17, but showing the aspherical lens changed after a second etching process has been performed; and  
         [0030]    [0030]FIG. 19 is similar to FIG. 18, but showing the completed aspherical lens in accordance with the second embodiment of the present invention after residual portions of the third photoresist film have been removed. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    Referring to FIG. 13, a first embodiment of an aspherical lens of the present invention is shown. The aspherical lens  10  has an aspherical surface  101  and an opposite flat surface  102 . A multiple-step grating (not labeled) is defined in the flat surface  102 . The grating has a plurality of nonsymmetrical steps (not labeled). In particular, the grating has “n” sections of uniform lengths ( 102 T shown in FIG. 13). A plurality of nonsymmetrical steps is defined along each uniform length  102 T. A height of each step is  102   h , and a width of each step is  102   b . The height  102   h  can be less than 100 nanometers.  
         [0032]    FIGS.  1  to  13  show a method for fabricating the aspherical lens  10 . The method comprises the following steps:  
         [0033]    (a) providing an aspherical lens  10 ; and  
         [0034]    (b) defining a multiple-step grating in the aspherical lens  10 .  
         [0035]    Referring to FIG. 1, a schematic side elevation of the aspherical lens  10  in accordance with step (a) of the method of the present invention is shown. The aspherical lens  10  has an aspherical surface  101  and an opposite flat surface  102 . The flat surface  102  is formed by a mechanical polishing process or a chemical polishing process.  
         [0036]    FIGS.  2  to  13  show successive stages of step (b). Firstly, a first photoresist film  121  is formed on the flat surface  102 , as shown in FIG. 3. In the illustrated embodiment, the first photoresist film  121  can be a positive photoresist film. Alternatively, the first photoresist film  121  can be a negative photoresist film. Secondly, a first mask  111  (shown in FIG. 2) is placed on the first photoresist film  121 . A size of the first mask  111  is the same as that of the flat surface  102 , and a resolution of the first mask  111  is 100 lines/mm. Furthermore, the first mask  111  is produced by a chemical etching method. A total area of exposure portions of the first mask  111  is half of a total area of the first mask  111 .  
         [0037]    Thirdly, a grating with a step is formed in the flat surface  102  by first exposure, developing and etching processes. FIG. 4 is a schematic cross-sectional view showing a first exposure process being performed on the aspherical lens  10  having the first mask  111  placed thereon. The first mask  111  is placed on the first photoresist film  121 , and preferably an ultraviolet radiation exposure process is executed by an aligner or a stepper. Alternatively, an exposure process can be executed by an electron beam writer.  
         [0038]    Then the photoresist portions of the first photoresist film  121  corresponding to the exposed areas are developed (see FIG. 5). The first photoresist film  121  is immersed in a developer. Because the first photoresist film  121  is a positive photoresist film, the portions of the first photoresist film  121  that have been exposed are dissolved. Alternatively, if the first photoresist film  121  is a negative photoresist film, the portions of the first photoresist film  121  that have not been exposed are dissolved.  
         [0039]    [0039]FIG. 6 shows the aspherical lens  10  after a first etching process has been performed. The first etching process is executed by using a photo etching technology or a μ-ray developing technology. The first etching process is controlled by a computer, so that it is performed only on those portions of the flat surface  102  that have been developed. Thus, a plurality of recesses is produced, with a depth of the recesses being determined by the steps desired in the fully completed grating.  
         [0040]    Referring to FIG. 7, the residual portions of the first photo resist film  121  are removed by eroding the flat surface  102  using a potassium cyanide solution, or by using another suitable technique. Thus, a precursor grating with a set of double steps is formed on the flat surface  102 .  
         [0041]    Referring to FIG. 8, a second mask  112  used for producing a multi-step grating is shown. A resolution of the second mask  112  is 200 lines/mm. Referring to FIGS. 9-12, a second photo resist film  122  is formed on the flat surface  102  of the aspherical lens  10 , and the second mask  112  is placed on the second photo resist film  122 . Second exposure, developing and etching processes are respectively performed, in similar fashion to the first exposure, developing and etching processes described above. A depth of recesses produced by the second etching process is half that of the recesses produced by the first etching process. Finally, residual portions of the second photo resist film  122  are removed by eroding the flat surface  102  using a potassium cyanide solution, or by using another suitable technique. Thus, the aspherical lens  10  having the grating with the plurality of nonsymmetrical steps formed thereat is obtained.  
         [0042]    By essentially repeating the above-described steps “n” times, each time using a mask having a resolution of (2 n )*(R) (R denoting a resolution of the first mask  111 , and n being a positive integer), and by controlling a depth of the recesses produced in each etching process to be half that of the recesses produced in the respective immediately preceding etching process, a 2 n+1 -step grating can be formed.  
         [0043]    Referring to FIG. 19, a second embodiment of the aspherical lens of the present invention is shown. The aspherical lens  10 ′ has an aspherical surface  101 ′ and an opposite flat surface  102 ′. A multiple-step grating (not labeled) is defined in the flat surface  102 ′. The grating has a plurality of symmetrical steps (not labeled). In particular, the grating has “n” sections of uniform lengths ( 102 T shown in FIG. 13). A plurality of symmetrical steps is defined along each uniform length  102 T. A height of each step is  102   h , and a width of each step is  102   b . The height  102   h  can be less than 100 nanometers.  
         [0044]    A method for fabricating the aspherical lens  10 ′ is as follows. Referring to FIGS. 1-7, according to the first embodiment, a grating with a set of double steps is formed on the flat surface  102 ′.  
         [0045]    Referring to FIG. 14, a third mask  113 ′ used for producing a multi-step grating is shown. A resolution of the third mask  113 ′ is 300 lines/mm. Referring to FIGS. 15-18, a third photo resist film  123 ′ is formed on the flat surface  102 ′ of the aspherical lens  10 ′, and second exposure, developing and etching processes are respectively performed. A depth of recesses produced in the second etching process is half that of the recesses produced in the first etching process. Finally, residual portions of the third photo resist film  123 ′ are removed by eroding the flat surface  102 ′ using a potassium cyanide solution, or by using another suitable technique. Thus, the aspherical lens  10 ′ having the grating with the plurality of symmetrical steps formed thereat is obtained.  
         [0046]    It is understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.