Abstract:
A method of manufacturing a microlens array requires at least two fiducial marks formed on a surface of a transparent medium opposite the microlens array. Additional optical features formed on the transparent medium adjacent the microlens array enables precise locationing of fiducial marks on an opposing surface when such surface is exposed to a collimated beam of light. The location of fiducial marks using the method of the invention is about 1 micron or less.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to U.S. application Ser. No. 10/027,994, filed Dec. 20, 2001, by Border, et al., and entitled, “Method Of Forming Fiducial Marks On A Micro-Sized Article;” U.S. application Ser. No. 10/027,834, filed Dec. 20, 2001, by Border, et al., and entitled, “Microlens Array;” U.S. application Ser. No. 10/027,863, filed Dec. 20, 2001 by Border, et al., and entitled, “Double-Sided Microlens Array And Method Of Manufacturing Same;” U.S. application Ser. No. 10/028,035, filed Dec. 20, 2001, by Border, et al., and entitled, “Laser Array And Method Of Making Same;” and, U.S. application Ser. No. 10/027,748, filed Dec. 20, 2001, by Border, et al., and entitled, “Fiber Optic Array And Method Of Making Same.” 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the field of microlens lens arrays. More particularly, the invention concerns forming fiducial marks on optical articles that require precise alignment in an optical system containing the microlens array. 
     BACKGROUND OF THE INVENTION 
     Optical systems, such as imaging systems, telecommunications devices, micro-optical systems, micro-mechanical systems, etc., are typically constructed of several different lenses and optical articles to deliver the desired optical performance. To avoid large overall losses in the optical system, the alignment of each lens and optical article with subsequent lenses and optical articles must be very precise. Fiducial marks are often created on the lenses and optical articles outside the optical area to serve as a reference point during alignment. Fiducial marks are particularly important in the case of aspheric lenses and lens arrays where it is difficult to identify the center of the lens during alignment activities. Fiducial marks are also very important for fiber optic arrays and laser arrays where multiple features dictate the need for a shared alignment reference which is located precisely in relation to all the optical features. As optical systems get smaller for fiber optics applications, like telecommunications and optical sensors, the need increases for precise alignment of the optical components and the accuracy of the associated fiducial marks. Alignment specifications of two (2) microns are now common with a desire to deliver submicron alignment accuracy. Consequently, the fiducial marks must be located with an accuracy of 1 micron or better. 
     Fiducial marks are well known in the semi-conductor manufacturing industry as an important tool for making multilayer semiconductors. In this case, the fiducial marks are incorporated as part of the semiconductor circuit plan. Due to the thinness (50-100 micron) of the semiconductor layers used in making multilayer semiconductors, the fiducial marks of multiple semiconductor layers can be viewed simultaneously using a high magnification microscope. The high magnification microscope aids in positioning the fiducial marks of one semiconductor layer over the fiducial marks of another semiconductor layer during the alignment process. 
     Forming fiducial marks in optical articles raises special challenges in that optical surfaces are typically relatively thick, often well over a 1000 micron in thickness. This is the case even in a microlens array that has microlenses that are well under a millimeter in diameter. The thickness of the microlens array makes it virtually impossible to accurately locate a fiducial mark by looking through the microlens array due to optical limitations. On the one hand, the location accuracy of the fiducial mark relative to the optical article is limited because the fiducial mark is displaced by refracted light passing through the microlens array material. Moreover, the thickness of the microlens array limits how close the microscope used for identifying the microlens array can be positioned to the fiducial mark. Consequently, only lower magnification microscopes can be used to look at the fiducial. Therefore, for optical articles, a method of applying a very accurately located fiducial mark on the side opposite to the optical article is needed. 
     In U.S. Pat. No. 6,005,294, by Tsuji et al., Dec. 21, 1999, entitled “Method Of Arranging Alignment Marks,” a method of making semiconductor devices uses multiple fiducial marks in such a way that the area occupied by the fiducial marks is reduced and the manufacturing productivity is correspondingly increased. While this patent does describe the state of the art for making semiconductor devices, the alignment process described therein is not appropriate for optical articles like lens arrays. As mentioned, in lens arrays, the significant thickness of the various lenses makes it impossible to view fiducial marks from multiple optical articles simultaneously due to the separation distance imparted by the material thickness of the lenses. 
     Also, U.S. Pat. No. 5,850,276, by Ochi et al., Dec. 15, 1998, entitled “Method Of Making LCD Device Having Alignment Mark Made Of Same Material And Formed At Same Time As Microlenses” and U.S. Pat. No. 5,771,085, by Ochi et al., Jun. 23, 1998, entitled “LCD Device With an Alignment Mark Having Same Material As Microlenses” each describe a process for molding fiducial marks into a microlens screen used for liquid crystal display devices. In these patents the shapes of the fiducial marks are also described in detail. The fiducial marks as described are protrusions in the shape of a cross or several other variations, located on the same side as the microlenses. The protrusions can be semicircular in cross section or another shape as long as the grooves between the protrusions stand out as dark lines when viewed with a reflecting microscope. The references recognize that lens characteristics, such as thickness, interfere with the ability to identify underlying fiducial marks. Further, the references show some appreciation for useful geometries of fiducial marks and for fiducial marks molded along with a microlens array. However, neither of the patents show appreciation for fiducial marks applied on the side opposite the optical surfaces in the microlens array. Furthermore, there is no appreciation by either of the references that advantages can be gained with a molded fiducial mark having lens characteristics. 
     Moreover, U.S. Pat. No. 6,096,155, by Harden et al., Aug. 1, 2000, entitled “Method Of Dicing Wafer Level Integrated Multiple Optical Elements” discloses the use of fiducials to aid in alignment of microlenses on wafers during the bonding of multiple wafers together prior to dicing. This patent generally teaches making integrated multiple optical elements with features to help control the thickness of adhesives and solders used to bond together the wafers. While effective use of the fiducial marks is described, there is absolutely no mention of ways to improve alignment of fiducial marks on one side with the optical element on the other side of the wafer. The techniques of embossing and molding fiducial marks, described in the patent, both suffer from locational inaccuracies from one side to the other o the order of plus or minus ten (10) microns. In molded microlenses and microlens arrays this inaccuracy is not acceptable. 
     Furthermore, U.S. Pat. No. 4,598,039, by Fischer et al., Jul. 1, 1986, entitled “Formation Of Features In Optical Material” describes the use of a laser to remove optical material in a controlled fashion. The laser can be used directly on the optical material or a layer of ablative absorber material can be put onto the surface of the optical material to enhance the coupling to the laser. This ablative technique is well suited to making fiducial type marks for alignment. However, the reference does not show appreciation for how to align the laser with a lens array that is located on the opposite side from the desired location for the fiducial marks. 
     Therefore, a need persists in the art for a method of forming fiducial marks onto optical articles and optical arrays on a surface opposite the optical article surface that enables precise alignment of the articles and optical arrays. Moreover, there is a compelling need for a special optical feature molded along with optical surfaces to focus light onto an opposing surface of the optical article or optical array thus enabling the formation of a fiducial mark onto the opposing surface with great accuracy. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the invention to provide a method for making fiducial marks on optical articles where the fiducial mark is located on a surface opposite the surface of the optical article. 
     It is a further object of the invention to utilize an optical feature made in conjunction with the optical article that focuses a high intensity beam of light onto the surface opposite the surface of optical article to thereby form a fiducial mark. 
     To accomplish these and other objects, features and advantages of the invention, there is provided, in one aspect of the invention, a method of manufacturing a microlens array having at least two fiducial marks arranged on a surface opposite the microlens array includes providing a transparent medium for mountably supporting the microlens array. In this embodiment, the transparent medium has a first surface for supporting the microlens array and a second surface opposite the first surface for receiving fiducial marks. A microlens array is formed on the first surface of the transparent medium and first and second optical features are formed on the first surface of the transparent medium adjacent to the microlens array. So as to distinguish the fiducial marks, at least a portion of the second surface of the transparent medium is altered before forming the fiducial marks thereon. In this embodiment of the invention, at least two fiducial marks are then formed on the altered portion of the second surface corresponding precisely to each one of the first and second optical features. 
     Consequently, the present invention has numerous advantages over prior art developments, including: it results in precision locating of fiducial marks it is a far superior method of aligning optical articles in an array, and, it is significantly easier to implement since all required optical features are formed with the same forming process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing as well as other objects, features and advantages of this invention will become more apparent from the appended Figures, wherein like reference numerals denote like elements, and wherein: 
     FIG. 1 a  is a perspective view of a prior art lens array with fiducial marks located on the side opposite the lens surfaces; 
     FIG. 1 b  is a perspective view of an optical system made in accordance with the method of the invention; 
     FIG. 2 is an elevated side view of a microlens array set in a transparent medium having optical features formed in accordance with the method of the invention; 
     FIG. 3 is an elevated side view of a microlens array set in a transparent medium with a fiducial mark forming means arranged for forming fiducial marks on an opposing surface of the transparent medium; 
     FIG. 4 is a perspective view of the optical article having generally circular fiducial marks on a surface opposite the optical article; 
     FIG. 5 is a perspective view of the optical article of the invention having a generally linear crossed fiducial mark on a surface opposite the optical article; 
     FIGS. 6 a  and  6   b  are perspective views of an alternative embodiment of the invention having a plurality of optical articles on either face of the transparent medium with corresponding fiducial marks on opposing surfaces in the transparent medium opposite the optical article; 
     FIG. 7 is a perspective view of an alternative embodiment of the invention comprising a laser array; and, 
     FIG. 8 is a perspective view of another embodiment of the invention comprising a fiber optic array. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A typical prior art microlens array  2  is illustrated in FIG. 1 a  for comparative purposes. According to FIG. 1 a , microlens array  2  has multiple microlenses  1  mounted coincidentally on a mounting flange  3 . Fiducial marks  7  are located on a surface  5  of mounting flange  3  opposite the surface  6  of microlenses  1 . Fiducial marks  7  would either be directly molded onto surface  5  or would be applied after referencing an edge of the optical surface from the opposite side of mounting flange  3 . In the case of direct molding of the fiducial marks  7 , mold misalignment due to clearance in the alignment pins across the molded parting line would limit the accuracy of fiducial mark  7  to approximately 15 microns or more. Using the edge referencing technique, experience has taught that each measurement introduces approximately 2-5 microns of inaccuracy. Since a minimum of three (3) measurements are required to identify an edge of a round lens, the total inaccuracy is a minimum of 6-15 microns to place the fiducial mark  7 . While this inaccuracy is usually acceptable for large optical articles, as the size of optics for applications such as fiber optics shrinks below 1000 micron, the alignment accuracy required shrinks as well. Consequently, it is not uncommon for alignment accuracy of microlenses to be 5 microns or better with some applications calling for 2 micron alignment. Obviously, the accuracy of the fiducial marks  7  must be better than the alignment accuracy required. 
     Turning now to FIG. 1 b , fiducial marks  13  formed in an optical article array, such as refractive lens array  11 , using the method of the invention is illustrated. In this embodiment, fiducial marks  13  arm used to align an optical assemblage  8  comprising refractive lens array  11  and laser array  9 . As described in the invention, additional optical features  16  are used to create fiducial marks  13  through optical means. According to FIG. 1 b , fiducial marks  13  on the lens array  11  are precisely located on opposing surface  11   b  of lens array  11 . To ensure precise alignment of optical assemblage  8 , each one of a Plurality of precision through-holes  15  formed in laser array  9  is alignably centered over a corresponding fiducial mark  13  in lens array  11 . This process aligns each of the lasers  9   a  in the laser array  9  with a refractive lens  11   a  in the refractive lens array  11 . After the optical assemblage  8  is aligned, it is rigidly affixed typically by potting in a suitable adhesive material. Precise alignment of precision through-holes is over the fiducial marks  13  is accomplished with a high power microscope (not shown) often with a computerized vision system linked to a computerized positioning system to automate the process. 
     Referring to FIGS. 2 and 3, an optical array  10  having accurately located fiducial marks  24 ,  28  formed on an opposing surface  30  of a transparent substrate  12  is illustrated. According to FIGS. 2 and 3, optical articles, such as microlens array  22 ,  32 , are supported on mounting surface  14  of transparent substrate  12  that is opposite surface  30 . Important to the invention, an additional optical feature  20  (described below) is formed adjacent to the microlens array  22 ,  32  to aid in precisely forming fiducial marks  25 ,  29  at focal points  24 ,  28 . According to FIG. 2, focal point  24  (corresponding to a fiducial mark  25 ) is then produced with a high intensity collimated beam of light  26 , As shown in FIG. 3, a laser source  27  may be used to produce such high intensity light  26 . The additional optical feature  20  receives the collimated beam of light  26  from laser source  27  and precisely focuses it  28  onto opposing surface  30  of the microlens array  10 . It is also important to the invention that prior to forming the fiducial marks  25 ,  29  at focal points  24 ,  28 , surface  30  of the transparent substrate  12  is altered or treated in the area  31  where the fiducial marts  25 ,  29  are to be formed. The objective of altering or treating surface  30  is to make suitably visible fiducial marks  13  when exposed to the focused high intensity light  26 . Suitable surface altering techniques include dip coating, roughening, spin coating, vacuum coating, metallizing, among others. 
     Skilled artisans will appreciate that there are several processes that may be used for forming a mold for making optical articles, such as optical array  10 , which includes additional optical features  20  as described. Such processes include lithographic printing, inkjet printing, indentation, diamond turning and diamond milling, each of which can deliver a position to position accuracy of 0.25 micron. Importantly, the method of the present invention uniquely uses the process for forming the microlens array  32  for also forming the additional optical features  20  that precisely locates the fiducial marks  25 ,  29  at focal points  24 ,  28 . 
     Referring to FIGS. 4 and 5, optical features having a variety of configurations with refractive or diffractive lenses can be used to create various shaped fiducial marks. According to FIG. 4, a lens array  40  has a plurality of lenses  41  formed on first surface  46  of transparent medium  44 . Generally round refractive lens feature  45  can be used to make a generally round fiducial mark  42  on the treated portion  49  of second surface  48  of transparent medium  44 , opposite first surface  46  of the transparent medium  44 . Moreover, to produce a generally linear fiducial mark, a generally linear lens feature is required (not shown). According to FIG. 5, a generally crossed linear refractive lens feature  50  is used to produce a generally crossed-shaped (X-shaped) fiducial mark  52 . Those skilled in the art will now appreciate that other patterns for the optical feature can be produced by a combination of refractive and diffractive optical features. 
     Referring to FIGS. 6 a  and  6   b , in another embodiment of the invention, double-sided optical arrays  58 ,  59  are illustrated. According to FIG. 6 a , double-sided optical array  58  has an arrangement of optical articles  60 ,  62  on either of opposing surfaces  61   a ,  61   b  in transparent medium  61 . Fiducial marks  69 ,  66  are formed on the treated portions  63 ,  64  of the opposing surfaces  61   a ,  61   b , respectively, by repeating the fiducial marking process described hereinabove. According to FIG. 6 b , alternatively, double-sided optical array  59  has optical features  72 ,  80  mounted on opposing surfaces  70   a ,  70   b  of transparent medium  70 . In this embodiment, two sets of fiducial marks  78 ,  83  are formed only on the treated portion  76  of surface  70   b  opposite surface  70   a  so the misalignment between the two optical articles  72 ,  80  could be easily determined. 
     Referring again to FIG. 6 a , double-sided optical array  58 , more particularly, has a first plurality of lenses  60  matched to a second plurality of lenses  62 , both being mounted on opposing surfaces  61   a ,  61   b  of transparent medium  61 . Two complimentary sets of additional optical features  65 ,  68  are formed in either of opposing surfaces  61   a ,  61   b , respectively. Optical features  65 ,  68  are used to form fiducial marks  66 ,  69  on the opposing surfaces  61   b ,  61   a , respectively. As shown in FIG. 6 a , optical feature  65  has a generally round shape which forms a generally round shaped fiducial mark  66  on the opposing suffice  61   b . In the same alternative, double-sided optical array  58 , a generally ring shaped optical feature  68  formed on surface  61   b  produces a generally ring shaped fiducial mark  69 . Alternatively, fiducial marks  66 ,  69  and optical features  68 ,  65  can be used as matching reference marks to measure the relative alignment of the optical articles  60 ,  62  on surfaces  61   a ,  61   b  by measuring the relative centering of the fiducial marks  66 ,  69  from the optical features  65 ,  68 . 
     It is the experience of the inventors that by using both refractive and diffractive lenses in the additional optical lens features, a wide variety of fiducial mark shapes can be created to fit different requirements. The additional optical lens feature can also be designed for different wavelengths if the fiducial marking is to be done using a light source that operates at a different wavelength than used by the optical array. 
     Referring again to FIG. 6 b , another embodiment of a double-sided optical array  59  is illustrated. As described above, a first plurality of lenses  72  in optical array  59  has additional generally round optical features  74  formed on surface  70   a  of transparent substrate  70 . Optical features  74  provide precise focusing of the collimated beam of light (FIG. 2) onto opposing surface  70   b  which forms a generally round fiducial mark  78  on a treated portion  76  of opposing surface  70   b . In this embodiment, the second plurality of lenses  80  is formed on opposing surface  70   b  of transparent medium  70 . Further, generally square fiducial marks  83  have been produced along with lenses  80  on the treated portion  76  of surface  70   b . The alignment of the first plurality of lenses  72  to the second plurality of leases  80  is preferably determined by measuring the magnitude and direction of the de-centering, i.e., the distance from an imaginary centerline passing through the leases to the fiducial mark of fiducial Mark  78  to fiducial mark  83 . 
     FIGS. 7 and 8, two additional embodiments of the invention are illustrated. According to FIG. 7, a laser array  110 , having lasers  90 , includes two additional optical features or crossed linear lenses  92  that produce fiducial marks  94  in the form of a cross (X) on the treated portion  97  of opposing surface  96   b . Lasers  90  may be arranged in openings in transparent medium  96  or they may be bonded to first surface  96   a  of transparent medium  96 . According to FIG. 8, a fiber optic array  120 , having fiber optic units  100  formed in transparent substrate  106 , includes additional optical features  102  adjacent to fiber optic units  100  that are used to produce fiducial marks  104  on the treated portion  107  of opposing surface  106   b  of the fiber optic array  120 . The fiber optic units  100  may be formed in transparent substrate  106  or they may be bonded to first surface  106   a . The same process, described above, for forming fiducial marks  94 ,  104 , is used in the present embodiments of the invention. 
     The invention has been described with reference to various embodiments thereof. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. 
     PARTS LIST 
       1  microlens 
       2  prior art microlens array 
       3  mounting flange 
       5  surface of mounting flange  3   
       6  surface of mounting flange  3  supporting microlens  1   
       7  fiducial marks on opposing surface  5   
       8  optical assemblage 
       9  laser array 
       9   a  lasers in laser array  9   
       10  optical array 
       11  refractive lens array 
       11   a  refractive lens in refractive lens array  11   
       11   b  surface for fiducial marks 
       12  transparent substrate 
       13  fiducial marks for refractive lens array  11   
       14  mounting surface 
       16  additional optical feature used to form fiducial marks  13   
       15  precision through-holes 
       20  additional optical feature 
       22  microlens array 
       24  focal point of additional optical feature  20  on an opposite surface to the microlens array 
       25  fiducial mark 
       26  high intensity collimated beam of light 
       27  laser source 
       28  focal point produced by the collimated light  26  passing through the additional optical features  20   
       29  fiducial mark 
       30  fiducial marking area on the opposite side of the microlens array  22   
       31  treated area on surface  30   
       32  multiple lens refractive lens array 
       40  lens array 
       41  plurality of lenses 
       42  generally round fiducial mark 
       44  transparent medium 
       45  generally round refractive lens feature 
       46  first surface of transparent medium  44   
       48  second surface of transparent medium  44   
       49  treated area of surface  48   
       50  crossed linear refractive lens feature 
       52  cross-shaped fiducial mark 
       58  alternative double-sided optical array 
       59  alternative double-sided optical array 
       60  optical articles (first plurality of lenses in optical array  58 ) 
       61  transparent medium 
       61   a,b  opposing surfaces in transparent medium  61   
       62  optical articles (second plurality of lenses in optical array  58 ) 
       63  treated portion of surface  61   a    
       64  treated portion of surface  61   b    
       65  round upper additional optical feature 
       66  found fiducial mark 
       68  ring-shaped lower additional optical feature 
       69  ring-shaped fiducial mark 
       70  transparent medium 
       70   a,b  opposing surfaces in transparent medium  70   
       72  optical features (first plurality of lenses in lens array  59 ) 
       74  round additional optical feature 
       76  treated portion of opposing surface  70   b    
       78  round spot fiducial marks 
       80  optical features (second plurality of lenses in lens array  59 ) 
       83  square fiducial mark 
       90  lasers 
       92  crossed linear lens on laser array  110   
       94  X-shaped fiducial marks on lower surface 
       96  transparent medium of laser array  110   
       96   a  first surface of transparent medium  96   
       96   b  second surface of transparent medium  96   
       97  treated portion of surface  96   b    
       100  fiber optic units 
       102  additional optical feature 
       104  X-shaped fiducial marks on lower surface 
       106  transparent medium of fiber optic array  120   
       106   a  first surface of transparent medium  106   
       106   b  opposing surface of transparent medium  106   
       107  treated portion of surface  106   b    
       110  laser array 
       120  fiber optic array