Abstract:
Micro-optical elements such as lenses and wave-guides are manufactured by printing a hardenable optical fluid using digitally driven ink-jet technology. An array of micro-optical elements are precisely positioned in an electroformed substrate having a surface containing structural openings which serve as molds for micro-droplets of optical fluids deposited from an ink-jet printhead. The structural openings have a stepped down edge, a shelf-like support surface below the edge and a centered aperture in the substrate. The micro-optical element formed is controlled by the shape of the edge in the surface of the substrate and the radius by the volume of micro-droplets deposited into the structural opening. The structural openings can be circular, or any desired shape which is easily and precisely formed in an electroformed substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of Provisional Application 60/185,521, filed Feb. 28, 2000 by the same inventor for which priority benefit is claimed. 
     CROSS-REFERENCE TO RELATED PATENT 
     This application relates to U.S. Pat. No. 5,498,444, titled “Method for Producing Micro-Optical Components” issued Mar. 12, 1996 to Donald J. Hayes, and U.S. Pat. No. 5,707,684, titled “Method for Producing Micro-Optical Components” issued Jan. 13, 1998 to Donald J. Hayes and W. Royall Cox, both patents being incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of making arrays of micro-optical elements precisely located and having specific optical shapes. 
     2. Background of the Invention 
     Micro-optical element arrays are primarily used in the optical communication and optical imaging fields. In general, these applications require that the optical elements have several features. They require control over the shape of the individual elements; the elements must be precisely located relative to each other and other optical components; the optical properties of the elements must be precisely controlled; the elements must be alignable with other optical components; and unwanted optical beams must be blocked and optical cross-talk limited. 
     There are various method s of making micro-optical elements such as molding, photolithography, and MicroJet printing. However, MicroJet printing is particularly advantageous as to the type of micro-optical elements that can be created and it permits accurate placement of elements within arrays. 
     Unlike the other methods, the present invention meets all the requirements a precision array demands, it: allows for the creation of specific optical shapes, controls precisely the optical element location, forms an aperture to block unwanted light, allows for the alignment of other optical elements such as optical fibers, and it can use a wide range of optical materials for its manufacture. 
     SUMMARY OF THE INVENTION 
     This invention provides, for the first time, an inexpensive way of creating micro-optical elements, by utilizing the ink-jet printing method of dispensing optical material for automated, in-situ fabrication of micro-element arrays. The flexibility of this data-driven method also enables variation of the shape of the printed micro-optical element. 
     The first step in fabricating a micro-optical element by means of inkjet printing comprises providing a substrate. The substrate has a surface with at least one structural opening defined by an edge in the surface leading into a sup port surface. The substrate is preferably an electroform comprising nickel and the structural openings are arranged as an array. The support surface has an aperture through the substrate which is positioned centrally with respect to the edge. The edge is preferably 1 to 5 microns deep so as to define the shape of the micro-optical element. The next step is to provide a digitally-driven printhead containing a hardenable optical fluid suitable for serving as a micro-optical element, preferably an ultraviolet (UV) light-curable epoxy, ejected in response to control signals. Micro-droplets of the optical fluid are deposited into the structural opening of the substrate, preferably centrally over the aperture but if the diameter of the micro-droplets is smaller than the aperture diameter, deposition is preferable over the support surface of the structural opening. In a preferred embodiment, the printhead moves over the surface of the substrate to deposit the optical fluid. The structural opening is then filled until a desired micro-optical element is formed where the element may have a radiused upper or lower surfaces, preferably both. The last step of the process is the hardening of the element, such as by UV light when UV light-curable epoxy is used in a preferred embodiment. Other means for curing such as by heat are also contemplated. 
     In a preferred embodiment, the production of an array of micro-optical lens elements is described. The first step in fabricating a micro-optical element by means of ink-jet printing comprises providing a substrate. The substrate has a surface with at least one circular structural opening defined by an edge in the surface leading into a support surface. The substrate is preferably an electroform comprising nickel and the structural openings are arranged as an array. The support surface has a circular aperture through the substrate which is positioned centrally with respect to the edge. The edge is preferably 1 to 5 microns deep so as to define and control the shape of the micro-optical element. The next step is to provide a digitally-driven printhead containing a hardenable optical fluid suitable for serving as a micro-optical element, preferably an ultraviolet (UV) light-curable epoxy, ejected in response to control signals. Micro-droplets of the optical fluid are deposited into the structural opening of the substrate. In a preferred embodiment, the printhead moves over the surface of the substrate to deposit the optical fluid. The circular structural opening is then filled until a desired micro-optical element profile is formed where the element may have a radiused upper or lower surface, preferably both. The last step of the process is the hardening of the element. 
     In another embodiment, the production of an array of elongated micro-optical elements in the form of waveguides is described. The first step in fabricating a micro-optical element by means of ink-jet printing comprises providing a substrate. The substrate has a surface with at least one elongated structural opening defined by an edge in the surface leading into a support surface. The substrate is preferably an electroform comprising nickel and the elongated structural openings are arranged as an array. The support surface has an aperture through the substrate which is positioned centrally with respect to the edge. The edge is preferably 1 to 5 microns deep so as to define the shape of the micro-optical element. The next step is to provide a digitally-driven printhead containing a hardenable optical fluid suitable for serving as a micro-optical element, preferably an ultraviolet (UV) light-curable epoxy, ejected in response to control signals. In a preferred embodiment, the printhead moves over the surface of the substrate to deposit the optical fluid. Micro-droplets of the optical fluid are deposited into the structural opening of the substrate, preferably centrally over the elongated aperture but if the diameter of the micro-droplets is smaller than the aperture diameter, deposition is preferable over the support surface of the structural opening. The elongated structural opening is then filled until a desired micro-optical element is formed where the element may have a radiused upper or lower surfaces, preferably both. The last step of the process is the hardening of the element, such as by UV light when UV light-curable epoxy is used in a preferred embodiment. Other means for curing such as by heat is also contemplated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages and features of the invention will become more apparent with reference to the following detailed description of presently preferred embodiments thereof in connection with the accompanying drawings, wherein like reference numerals haven been applied to like elements, in which: 
     FIG. 1 a  is a schematic plan view showing an array of circular structural openings for producing an array of micro-optical elements according to the method of the present invention; 
     FIG. 1 b  is a side-view of the array on lines  1   b — 1   b  in FIG. 1 a.    
     FIG. 2 is a cross-sectional view of a structural opening from the array of FIG. 1 a  on the line  2 — 2  showing the relative proportions between the structural opening, support surface, edge, and the taper of the aperture of the substrate in an embodiment of the present invention. 
     FIG. 3 is a schematic side-view showing the relative position of the printhead relative to the substrate as it deposits micro-droplets of optical material into the structural openings according to the method of the present invention. 
     FIG. 4 is a cross-sectional view showing a micro-optical element formed in the structural openings of FIG. 3 after optical material was deposited and the structural opening filled according to a preferred embodiment of the method of the present invention. 
     FIG. 5 is a cross-sectional view showing another micro-optical element of a different radius formed in a structural opening after optical material was deposited and the structural opening filled according to a preferred embodiment of the present invention. 
     FIG. 6 is a schematic perspective side-view showing the placement of the printhead relative to an array of openings of a preferred embodiment of the present invention as it deposits micro-droplets of a optical material into an elongated structural opening. 
     FIG. 7 is a side view on lines  7 — 7  in FIG. 6 of the array of the embodiment showing placement of the printhead relative to an array of openings of the preferred embodiment of the present invention shown in FIG.  6 . 
     FIG. 8 a  is a plan view of an array of elongated structural openings according to a preferred embodiment of the present invention showing a close regular arrangement of structural openings. 
     FIG. 8 b  is a cross-sectional view of FIG. 8 a  on the lines  8   b — 8   b  showing the arrangement of structural openings, support surface, edge, and aperture of the substrate. 
     FIG. 9 is a schematic drawing of a substrate like FIG. 1 a  with alignment features. 
     FIG. 10 is a cross-sectional view showing the positioning of an optical fiber under the lower surface of the substrate in line with the aperture of the structural opening. 
     FIG. 11 a  is a schematic side-view of a mandrel coated with a layer of photoresist. 
     FIG. 11 b  is a schematic side-view of the mandrel of FIG. 11 a  after further process to produce photoresist patterns. 
     FIG. 11 c  is a schematic side-view of the mandrel of FIG. 11 b  with the photoresist patterns surrounded as shown with a suitable electroform material to form an electroform sheet which can be used in the process of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention applies ink-jet printing technology to the fabrication of arrays of micro-optical elements for optical fibers. Shown in FIG. 1 a  is a substrate  10  with an array of structural openings  14  used to make precision micro-optical elements. Although the structural openings of FIGS. 1 a  and  1   b  are circular, they represent only one embodiment of the present invention. Each structural opening  14  is defined by an edge  16  in the upper surface  18  of substrate  10 . Edge  16  leads into support surface  20  wherein an aperture  22  is positioned centrally with respect to the edge  16 . Edge  16  of structural opening  14  preferably has a depth of about 1 to about 5 microns. FIG. 1 b  is an endview of substrate  10  that illustrates the thin profile between upper surface  18  and lower surface  24  of the substrate. A magnified representation of single structural opening  14  of array  12  is shown in FIG. 2 before filling with optical material, which also reflects the tapered bottom surface  26  of the circular structural opening embodiment to be discussed below. The aperture being positioned centrally with respect to the edge means that the edge of the aperture opening is generally the same distance from the edge of the structural opening, i.e., the aperture is centered. 
     Substrate  10  is preferably an electroform plate, preferably made of nickel although any suitable material is appropriate. The electroform process itself is well known and will be described later. Structural openings  14  function as a mold in the fabrication of the micro-optical element and as an attachment point for a micro-optical element and an optical fiber. 
     FIG. 3 shows a digitally-driven printhead  28  depositing a predetermined size and number of micro-droplets  30  of optical fluid into structural openings  14  to form micro-optical elements  32 . Apertures  22  of unfilled structural openings  14  are seen with printhead  28  moving in the direction of the arrow to fill them. Methods of operating an ink-jet printhead to deposit optical polymeric materials in a fluid state are disclosed in U.S. Pat. Nos. 5,498,444 and 5,707,684 entitled Method for Producing Micro-Optical Components by the assignee hereof, the disclosures of which are incorporated by reference. Digitally-driven printhead  28  ejects micro-droplets  30  of optical fluid through orifice  34 . The diameter of orifice  34  is preferably between about 20 μm to about 120 μm although smaller or larger orifice diameters are acceptable. The printhead preferably includes a piezoelectric device operable in a drop-in-demand mode and is heatable to control the viscosity of the optical fluid. The movement of the printhead and substrate relative to each other is computer-controlled. The substrate is positioned on a computer-controlled stage movable in the X-Y plane. The computer moves the stage so that a structural opening is positioned to receive optical fluid micro-droplets  30  deposited by the digitally-driven printhead. Ejection of micro-droplets by the printhead is preferably controlled by the same computer. After filling one structural opening, the computer moves the substrate to position the next structural opening under the ejection orifice then activates the printhead to eject the micro-droplets into the structural opening. The stage is again repositioned so the next structural opening is positioned to receive micro-droplets deposited by the digitally-driven printhead and the printhead is again activated to deposit micro-droplets of optical fluid until micro-lenses are formed in each structural opening. 
     The optical fluid can be any material, or combination of materials, capable of forming a relatively transparent micro-optical element after hardening. Optical epoxies are an example. Some specific commercial materials which have been suitable for forming micro-optical elements include Summers Optical SK9 (Refractive Index 1.49) by Summers Optical, Inc., P.O. Box 162, Fort Washington, Pa. 19034; Norland No. NOA-73 (Refractive Index 1.56) by Norland Products, Inc., P.O. Box 7145, New Brunswick, N.J. 08902); and Epotek No. OG-146 (Refractive Index 1.48) by Epogy Technology, Inc., 14 Fortune Drive, Billerica, Mass. 01821. In a preferred embodiment of the invention, an ultraviolet (UV) light-curable epoxy is used. When used, the diameter of the optical epoxy micro-droplets is preferably within the range of about 8 μm to about 300 μm. Most typically the micro-droplets would be around 50 microns. 
     In a preferred embodiment a micro-optical lenslet element  32  formed in FIG. 3 is shown in FIG. 4 situated in structural opening  14  of substrate  10  wherein the structural opening is circular. Micro-optical element  32  has a first radiused outer surface  36  formed, in the shape of a hemisphere or a section of a sphere, above support surface  20 . A pedestal portion  40  or step-down  40  coincides with the height of edge  16  above support surface  20 . A second radiused surface  38  in the shape of a hemisphere or a section of a sphere is formed below support surface  20 . Bottom surface  24  slopes upward toward upper surface  18  to form a tapered wall portion  26  at aperture  22 . The tapered wall  26  is formed naturally in the process of making the electroformed substrate  10 . 
     As shown in FIG. 10, taper  26  can be used to center an optical fiber  60  at the bottom surface  24  of the substrate  10  under aperture  22  of a micro-lens  32 . The axis of the core  61  of optical fiber  60  is centered with respect to the central axis of lens  32 . Edge  16  and support surface  20  control the shape of the micro-optical material upon filling the structural opening  14  and edge  16  centers the material over aperture  22 . Micro-optical element  32  typically has a diameter which coincides with the diameter of structural opening  14 . Here the diameter is slightly greater because the micro-optical element  32  is spherical and higher than the hemispherical plane. The process produces lenses with spherical outer surfaces when the structural openings are circular. 
     In one aspect of the present invention best seen in FIG. 4, a number of micro-droplets  30  of micro-optical element lens material was deposited so that a micro-optical element  32  forms a first radiused surface  36  above support surface  20  and, in another embodiment, a second radiused surface  38  is formed below support surface  20 . However, it is to be understood that a micro-optical element lacking both or either a first-radiused surface or second-radiused surface could be formed according to the present invention. The radius of the lenslet being formed is controlled by varying the size or number of droplets of optical material that are deposited. The structural discontinuity at the edge  16  controls the shape (diameter) of the lens that is formed. 
     The role of edge  16  in forming a first radiused surface  36  and in centering micro-optical element  32  over aperture  22  is shown in FIG. 5 which reflects actual data. In FIG. 5, even though lens  32  was made larger than lens  32  in FIG. 4, it was still controlled by the edge discontinuity from spreading out uncontrollably over the surface  18 . The lenslet formed in FIG. 5 was made from an optical epoxy jetted from a digitally driven printhead at about 55° C. from fluid having a viscosity of 6 to 10 centipoise. The orifice in the electroform sheet was about 45 microns. The substrate in this case was held at room temperature. The circular structural openings can be closely and precisely spaced to result in formed micro-optical lens elements also being closely and precisely spaced. 
     FIG. 9 shows a substrate  11  with alignment holes  62 . Alignment holes  62  permit precise location of the structural openings and precise alignment of the array relative to the printhead  28  in FIG. 3 when forming the lenslets  32 . Moreover, alignment holes  62  allow for accurate positioning of the micro-optical element array  12  relative to other optical components. 
     Another embodiment of the present invention is shown in FIG.  6 . Elongated structural opening  46  is defined by edge  50  in upper surface  18  of substrate  13  leading into support surface  48 . Support surface  48  has an aperture  52  extending through the substrate and positioned centrally with respect to edge  50  and support surface  48 . Elongated structural opening  46  provides a method of making micro-optical elements such as waveguides of various configurations. Although the structural openings are shown as linear, they could also be curved for special applications. Substrate  13  is an electroform plate, preferably made of nickel although any suitable material is appropriate. The process of making the electroform substrate  13  is the same as for substrate  10 , except for the shape of the openings. Structural openings  46  function as a mold in the fabrication of the micro-optical lens element. The micro-optical element material  30  in FIG. 6 is ejected from digitally-driven printhead  28  and deposited in elongated structural openings  46  to form elongated micro-optical waveguide elements  44 . The elongated structural openings  46  in FIG. 6 are precisely distanced from one another. The micro-optical waveguide elements  44  are therefore also precisely distanced from each other as shown. 
     FIG. 7 shows a cross-section through substrate  13  of FIG. 6 reflecting the position of digitally-driven printhead  28  over an elongated structural opening  46  as droplets  30  are being ejected to form waveguides  44 . Digitally-driven printhead  28  can be positioned directly over aperture  52  when the diameter of micro-droplets  30  is greater than the diameter  58  of aperture  52 . 
     FIG. 8 a  shows an array of elongated structural openings  46  before filling with optical material. Thanks to the electroform process, elongated structural openings  46  can be positioned closely to each other in the array. The depth of structural openings  46  and edge  50  are preferably between about 1 μm to about 5 μm. 
     FIG. 8 b  shows the positioning of the array of elongated structural openings  46  in cross-section. Although FIGS. 6 through 8 a  illustrate micro-optical elements of uniform cross-sectional profile, the elements can be of varying cross-section as well. Since the electroform is essentially made by a photolithographic process, the structural openings can be varied in shape and size and reproduced exactly. 
     FIGS. 1 a - 11   c  illustrate one form of the electroform process to create an electroform tuba product used as the substrate  10 , 13  in the present invention. FIG. 11 a  shows a flat metal mandrel  64  coated with a layer of photoresist  66 . The photoresist layer is typically between from about 0.5 microns to about 5 microns thick. The photoresist layer is patterned with standard photolithography processes which are common in the semiconductor industry. After patterning and further processing, patterns  68  are left on the surface as shown in FIG. 11 b . Mandrel  64  is used as one of the electrodes in a plating process. A suitable metal, preferably nickel, is plated onto mandrel  64  and extends over the photoresist patterns  68  as shown in FIG. 11 c . Electroform plate  70  is peeled off the mandrel and photoresist layer  66  is then chemically removed. Although electroform products are described for use as substrates  10  and  13 , other substrate manufacturing methods such as chemical etching or stamping could also be employed. 
     Although the invention has been disclosed above with regard to a particular and preferred embodiment, it is not intended to limit the scope of the invention. For instance, although the inventive method has been set forth in a prescribed sequence of steps, it is understood that the disclosed sequence of steps may be varied. It will be appreciated that various modifications, alternatives, variations, etc., may be made without departing from the spirit and scope of the invention as defined in the appended claims.