Patent Publication Number: US-2006012880-A1

Title: Optical device with refractive and diffractive properties

Description:
BENEFIT CLAIM UNDER 35 U.S.C. § 120  
      This application is a continuation-in-part of prior U.S. Non-provisional Application for patent Ser. No. 10/137,630 filed on May 2, 2002. 
    
    
     BACKGROUND OF THE INVENTION  
      Microlens fabrication is an important technique in the quest to build compact fiber optical telecommunications devices capable of operating at terabit speeds. In these compact devices, the lenses that are used to align and focus incoming and outgoing light signals are becoming smaller and are being placed closer to miniature detectors or light sources, such as Vertical Cavity Surface Emitting Lasers (VCSELs).  
      Various types of microlens fabrication techniques have been used in the optical telecommunications industry, such as polymer stamping or molding processes and polymer reflow processes. However, the typical polymers used in the polymer stamping or molding processes and polymer reflow processes are low viscosity polymers that do not perform well at temperatures above 250° C. In applications where the assembly fabrication temperature may be in excess of 300° C., the optical properties of the microlens array may deteriorate due to shape deformation and material discoloration caused by the high fabrication temperatures. In addition, low viscosity polymers are typically not capable of producing thick lenses, which may be required depending upon the application. Furthermore, the lens shapes attainable by typical photoresist materials are limited by the surface tension of the photoresist in liquid form.  
      Therefore, what is needed is an economical lens fabrication technique that produces lenses capable of withstanding subsequent high processing temperatures and allows the lens shape and height to be controlled.  
     SUMMARY OF THE INVENTION  
      Embodiments in accordance with the invention provide an optical device including two discrete relief structures to provide refractive and diffractive properties. A first discrete relief structure has a curved surface to provide the refractive properties, and a second discrete relief structure disposed over the first discrete relief structure includes one or more layers of an optically-transparent polymer material to provide the diffractive properties. The one or more layers in the second discrete relief structure define a surface curvature envelope formed of discontinuous diffractive features that produce the diffractive properties of the optical device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The disclosed invention will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein:  
       FIG. 1  is a flowchart illustrating exemplary steps for fabricating a micro-optics device in accordance with embodiments of the invention;  
       FIGS. 2A-2G  are cross-sectional views illustrating the fabrication of a micro-optics device in accordance with one embodiment of the invention;  
       FIG. 3  is a flowchart illustrating exemplary steps for fabricating a micro-optics device in accordance with the embodiment shown in  FIGS. 2A-2G ;  
       FIGS. 4A-4F  are cross-sectional views illustrating the fabrication of a micro-optics device in accordance with another embodiment of the invention;  
       FIG. 5  is a flowchart illustrating exemplary steps for fabricating a micro-optics device in accordance with the embodiment shown in  FIGS. 4A-4F ;  
       FIGS. 6A-6I  are cross-sectional views illustrating the fabrication of a micro-optics device in accordance with another embodiment of the invention;  
       FIG. 7  is a flowchart illustrating exemplary steps for fabricating a micro-optics device in accordance with the embodiment shown in  FIGS. 6A-6I ;  
       FIG. 8  is a perspective view of an exemplary optical device with refractive and diffractive properties fabricated in accordance with embodiments of the present invention; and  
       FIG. 9  is a cross-sectional view of the optical device of  FIG. 8 . 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      As used herein, the term “resist” is defined as a polymer resist material that is transparent to optical wavelengths equal to or greater than 350 nm and that has a viscosity sufficient to allow stacking of layers. For example, the viscosity of the polymer resist material can be between 2,000 and 100,000 centipoise, 2,500 and 100,000 centipoise, 3,000 and 100,000 centipoise, 3,500 and 100,000 centipoise, 4,000 and 100,000 centipoise, 4,500 and 100,000 centipoise or 5,000 and 100,000 centipoise. The high viscosity (e.g., at or above 2,000 centipoise) of the resist material allows for thick films (up to mm range) to be produced, and therefore, thick lenses to be produced. Furthermore, the optical transparency of the resist material enables the resist to be used as a lens material and allows the thick films produced by the resist to be thermally cured down to the substrate.  
      In one embodiment, the resist is an epoxy-based polymer resist. Epoxy-based polymer resist materials are able to be flowed at low temperatures before the polymer becomes cross-linked and, after subsequent processing, the materials are stable at temperatures above 250° C. (i.e., the resist will not reflow during subsequent processing as many other polymers do). An example of an epoxy-based polymer resist is SU-8, which is a commercially available resist developed by IBM and sold by MicroChem Corporation. SU-8 becomes chemically inert and immovable once exposed to ultraviolet (UV) light and thermally cured.  
       FIG. 1  illustrates exemplary steps for fabricating a micro-optics device in accordance with embodiments of the invention. A layer of the resist is deposited onto a substrate (step  100 ) and patterned photolithographically to define a first lens layer (step  110 ). The substrate can be a substrate transparent to light within a particular range of wavelengths (e.g., visible, x-ray, infrared) and include one or more layers of an anti-reflection material, such as dielectric materials of appropriate optical indices and thicknesses. In other embodiments, for reflection optics applications, the substrate need not be transparent, and can include one or more layers of a reflection material, such as metallic materials and/or dielectric materials of appropriate optical indices and thicknesses.  
      To obtain the desired geometry of the micro-optics device, additional layers of the resist (step  120 ) can be deposited (step  100 ) and patterned photolithographically (step  110 ) to build a complete lens structure. A final smoothing layer of the resist can be deposited over the lens structure (step  130 ), patterned (step  140 ) and thermally cured (step  150 ) to provide a smooth surface for the micro-optic device. For example, the substrate can be placed either on a hot plate or in an oven at a temperature between 90° C. and 120° C. However, it should be understood that other temperatures may be used, depending upon the materials involved. The resulting micro-optics device can contain, for example, one or more of each of the following types of microlenses: concave lenses, convex lenses, circular lenses, elliptical shape lenses, prisms, Fresnel lenses, gratings and diffractive optics. Moreover, the micro-optics device fabrication technique enables easy integration of the micro-optics device into an assembly and allows the micro-optics device to be packaged together with other IC components economically.  
      In one embodiment, as shown in  FIGS. 2A-2G , a micro-optics device, such as an array of microlenses, is fabricated in a series of pattern steps. In each step, layer of an epoxy-based, negative-working, photo-definable polymer resist  210  is deposited on substrate  200 , such as quartz, glass, silicon, ceramic, plastic, flexible thin films and other types of materials. Resist  210  can be deposited using any known deposition process, such as, for example, spin-coating. An example of a spin-coating process is as follows: (1) place the substrate on a vacuum chuck; (2) dispense the resist over the substrate in a static mode; (3) spin the substrate up to a set speed (e.g., 500-5000 rpm); (4) maintain the set speed for certain period of time; and (5) ramp down the speed until the substrate stops spinning. During the spin cycle, the resist spreads and coats the surface of the substrate. Excess resist is spun off in order to produce the desired resist film thickness. The result of the deposition process is layer of resist  210  overlying substrate  200 , as shown in  FIG. 2A .  
      After deposition of layer of resist  210  onto substrate  200 , the edges of the lenses are defined photolithographically, as shown in  FIG. 2B . For example, in a standard photolithography process, resist  210  is exposed to ultraviolet (UV) light (e.g., 350 nm-400 nm) with a photo-mask at room temperature, and then baked at a typical temperature of between 95° C. and 120° C. (although other temperatures may be used, depending upon the materials involved). The UV light changes the property of exposed resist  210  to be easy or difficult to dissolve in a developer solution based on the tone of resist  210  (negative or positive tone). Negative-working polymer resist  210  shown in  FIG. 2B  becomes cross-linked (i.e., hard) in the exposed regions, and therefore, resistant to developer solution. The unexposed regions of resist  210  dissolve in the developer solution, leaving the desired pattern of one or more stacks  210   a  of the first layer of resist, as shown in  FIG. 2B . For example, in some embodiments, the developer solution can be propylene glycol monomethyl ether acetate (PGMEA). However, it should be understood that other developer solutions may be used, depending upon the materials involved.  
      In  FIG. 2C , second layer of resist  210  is shown deposited (e.g., spin-coated) over patterned stacks  210   a  in the first layer of resist. Second layer of resist  210  is also patterned photolithographically to define one or more stacks  210   b  of the second layer of resist overlying one or more of the stacks  210   a  of the first layer of resist. As shown in  FIG. 2D , second layer stacks  210   b  are smaller in area than first layer stacks  210   a  to create “pedestal” stacks of resist material. Subsequent layers of resist, not shown, can be deposited and defined in a stair case elevation pattern, where bottom pedestal  210   a  has the largest area and top pedestal  210   b  has the smallest area. The height and curvature of the lens is determined by the number of resist layers and the outer edge diameters of each resist layer.  
      As shown in  FIG. 2E , final smoothing layer of resist  210  is spin-coated over previous patterned stacks  210   a  and  210   b  of resist. It should be noted that final layer of resist  210  can have a variable viscosity that is less than the viscosity of other resist layers (e.g., less than 2,000 centipoise). Final layer of resist  210  is also exposed to UV light with the photo-mask used in defining the edges of the lenses for the first layer of resist and developed, such that final patterned layer of resist  210   c  covers all other stacks  210   a  and  210   b  of resist, as shown in  FIG. 2F . Resulting stack of resist layers  210   a ,  210   b  and  210   c  is thermally cured (i.e., soft baked) to allow final patterned layer  210   c  to flow smoothly over other resist stacks  210   a  and  210   b  to cover stacks  210   a  and  210   b  and fill in between stacks  210   a  and  210   b . Surface tension resulting from the thermal cure process pulls the shape of final patterned layer  210   c  of resist into lens  220  having a curved surface, as shown in  FIG. 2G . To finalize the shape and size of lens  220 , lens  220  is blanket exposed (i.e., no mask is used) with UV to cross-link the polymer material in order to harden lens  220 .  
      The fabrication process produces lithographically defined geometries in a polymer. For example, the fabrication process enables control of various lens parameters, such as the height, diameter and figure of the lens.  FIG. 2G  further illustrates several examples of lens parameters that are variable using the fabrication process described above. The parameters are as follows: curvature of a concave lens  220   a ; diameter of a concave lens  220   b ; curvature of a convex lens  220   c ; diameter of a convex lens  220   d ; and height of the lenses  220   e . However, it should be understood that the lens parameters capable of being controlled by the fabrication process of the invention are not limited to those shown in  FIG. 2G , but rather can be extended to any lens geometry.  
       FIG. 3  is a flowchart illustrating exemplary steps for fabricating a micro-optics device in accordance with the embodiment shown in  FIGS. 2A-2G . An initial layer of an epoxy-based negative-working photo-definable polymer resist is spin-coated onto a substrate (step  300 ). If desired, the substrate with the layer of resist thereon can be thermally cured (step  310 ) (i.e., soft baked) as a precursor to photolithography. In the initial photolithography step (step  320 ), the edges of the lenses are defined by exposing the resist to ultraviolet (UV) light (e.g., 350 nm-400 nm) with a photo-mask having an initial pattern masking (step  330 ), subjecting the resist to a post exposure bake (step  345 ) and dissolving away unexposed regions of the resist in a developer solution (step  350 ), leaving one or more stacks of resist material.  
      If additional layers of resist are to be applied (depending upon the desired height and curvature of the lens) (step  360 ), each additional layer of resist is spin-coated (step  300 ) over the previously defined stack(s) of resist, soft-baked (step  310 ) and photolithographically patterned using a photo-mask having a smaller pattern masking that is capable of defining one or more stacks of resist that are smaller in area than the immediately preceding stacks of resist and that overly one or more of the immediately preceding stacks of resist (step  335 ). The resist is then baked (step  345 ), and unexposed areas of resist are dissolved away in developer solution (step  350 ), leaving a stair case elevation pattern of “pedestals” of resist, where the bottom pedestal of resist has the largest area and the top pedestal of resist has the smallest area.  
      A final smoothing layer of resist (step  360 ) is spin-coated over the previous patterned stacks of resist (step  300 ) and soft-baked (step  310 ). The final layer (step  325 ) is also exposed to UV light with the initial photo-mask used in defining the edges of the lenses for the first layer of resist (step  340 ), subjected to a post exposure bake (step  345 ) and developed (step  350 ), such that the final patterned layer of resist covers all other layers of resist. The resulting stack of resist layers (step  360 ) is thermally cured (step  370 ) to allow the final layer to flow smoothly over the other resist layers to cover the layers and fill in between the layers. The surface tension of the melted final layer of resist pulls the final resist layer into a lens shape having a curved surface. To finalize the lens shape and size, the lens is blanket exposed (i.e., no mask is used) with UV to cross-link the polymer material in order to harden the lens (step  380 ). A final thermal treatment can be applied, if necessary, to cure the lenses further to improve performance in subsequent processing (step  390 ).  
      In another embodiment, as shown in  FIGS. 4A-4I , an alternate series of pattern steps can be used to fabricate a micro-optics device. First layer of an epoxy-based negative-working photo-definable polymer resist  210   a  is deposited on substrate  200 . First layer of resist  210   a  is exposed to ultraviolet (UV) light (e.g., 350 nm-400 nm) with a photo-mask to become cross-linked (i.e., hard) in exposed regions  215   a , and therefore, resistant to developer solution. In  FIG. 4B , second layer of resist  210   b  is shown deposited (e.g., spin-coated) over the first layer of resist. Second layer of resist  210   b  is also exposed to UV light using a photo-mask that allows smaller areas  215   b  of the second layer of resist to be exposed, as compared to first layer of resist  210   a . Subsequent layers of resist, not shown, can be deposited and exposed in a stair case elevation pattern, where exposed area  215   a  of bottom layer  210   a  has the largest area and exposed area  215   b  of top layer  210   b  has the smallest area. The unexposed regions of layers of resist  210   a  and  210   b  are dissolved together in the developer solution, leaving the desired pattern of one or more stacks  210   a  and  210   b  of resist, as shown in  FIG. 4C .  
      As shown in  FIG. 4D , final smoothing layer of resist  210  is spin-coated over previous patterned stacks  210   a  and  210   b  of resist. Final layer of resist  210  is also exposed to LV light with the photo-mask used in defining the edges of the lenses for the first layer of resist and developed, such that final patterned layer of resist  210   c  covers all other stacks  210   a  and  210   b  of resist, as shown in  FIG. 4E . Resulting stack of resist layers  210   a ,  210   b  and  210   c  is thermally cured (i.e., soft baked) to allow final patterned layer  210   c  to flow smoothly over other resist stacks  210   a  and  210   b  to cover stacks  210   a  and  210   b  and fill in between stacks  210   a  and  210   b . Surface tension resulting from the thermal cure process pulls the shape of final patterned layer  210   c  of resist into lens  220  having a curved surface, as shown in  FIG. 4F . To finalize the shape and size of lens  220 , lens  220  is blanket exposed (i.e., no mask is used) with UV to cross-link the polymer material in order to harden lens  220 .  
       FIG. 5  is a flowchart illustrating exemplary steps for fabricating a micro-optics device in accordance with the embodiment shown in  FIGS. 4A-4F . An initial layer of an epoxy-based negative-working photo-definable polymer resist is spin-coated onto a substrate (step  500 ). If desired, the substrate with the layer of resist thereon can be thermally cured (step  510 ) (i.e., soft baked) as a precursor to photolithography. In the initial photolithography step (step  520 ), the edges of the lenses are defined by exposing the resist to ultraviolet (UV) light (e.g., 350 nm-400 nm) with a photo-mask with an initial pattern masking (step  530 ) and baking the resist (step  540 ) to cross-link (i.e., harden) the resist in the exposed regions. If additional layers of resist are to be applied (depending upon the desired height and curvature of the lens) (step  545 ), each additional layer of resist is spin-coated (step  500 ) over the previously defined stack(s) of resist, soft-baked (step  510 ) and exposed to UV light using a photo-mask having a smaller pattern masking that is capable of defining one or more stacks of resist that are smaller in area than the immediately preceding stacks of resist and that overly one or more of the immediately preceding stacks of resist (step  535 ). The resist is baked (step  540 ), and unexposed areas of resist are dissolved away together in developer solution (step  550 ), leaving a stair case elevation pattern of “pedestals” of resist, where the bottom pedestal of resist has the largest area and the top pedestal of resist has the smallest area.  
      A final smoothing layer of resist is spin-coated over the previous patterned stacks of resist and soft-baked (step  560 ). The final layer is also exposed to UV light with the photo-mask used in defining the edges of the lenses for the first layer of resist (step  570 ), subjected to a post exposure bake (step  575 ) and developed (step  580 ), such that the final patterned layer of resist covers all other layers of resist. The resulting stack of resist layers is thermally cured (step  590 ) to allow the final layer to flow smoothly over the other resist layers to cover the layers and fill in between the layers. The surface tension of the melted final layer of resist pulls the final resist layer into a lens shape having a curved surface. To finalize the lens shape and size, the lens is blanket exposed (i.e., no mask is used) with UV to cross-link the polymer material in order to harden the lens (step  595 ). A final thermal treatment can be applied, if necessary, to cure the lenses further to improve performance in subsequent processing (step  598 ).  
      In a further embodiment, as shown in  FIGS. 6A-61 , a micro-optics device, such as an array of microlenses, is fabricated in a series of shell steps. As can be seen in  FIG. 6A , core layer of an epoxy-based negative-working photo-definable resist  210  is first deposited onto substrate  200 . Core layer of resist  210  is patterned photolithographically, as described above. The resulting pattern is one or more core stacks  210   a  of resist material, as shown in  FIG. 6B .  
      In  FIG. 6C , second layer of resist  210  is shown deposited (e.g., spin-coated) over defined core stacks  210   a  in the first layer of resist. Second layer of resist  210  is dissolved in developer solution without patterning (no UV exposure). Due to loading effects, spacers  210   d  of resist material are left at the base of each core stack  210   a , as shown in  FIG. 6D . However, it should be noted that in certain embodiments, spacer  210   d  resist material may not be needed, and therefore, the micro-optics device can be fabricated using the core layer of resist and any subsequent layer(s) as described below. Subsequent layers of resist  210  can be deposited (e.g., spin-coated) over defined stacks  210   a  (and spacers  210   d ) of resist, as shown in  FIG. 6E , and patterned photolithographically to define one or more shells  210   e  of resist overlying one or more stacks  210   a  (and spacers  210   d ) of resist. As shown in  FIG. 6F , each shell  210   e  of resist material has a larger area than the combination of stack  210   a  and spacers  210   d . The edges of resist shell  210   e  define the diameter of the lens. The number of shells  210   e  used depends upon the desired height, width and curvature of the lens.  
      As shown in  FIG. 6G , final smoothing layer of resist  210  is spin-coated over previous patterned shells  210   e  of resist and patterned photolithographically to define final shell  210   f  of resist, as shown in  FIG. 6H . Resulting shells  210   e  and  210   f  of resist are thermally cured (i.e., soft baked) to allow final shell  210   f  of resist to flow smoothly over other resist shells  210   e , and to allow surface tension resulting from the thermal cure process to pull the shape of final shell  210   f  of resist into lens  220  having a curved surface. To finalize the shape and size of lens  220 , as shown in  FIG. 61 , lens  220  is blanket exposed (i.e., no mask is used) with UV to cross-link the polymer material in order to harden lens  220 . By defining a series of shells  210   e  and  210   f , resulting lens  220  will have smooth round sidewalls with a hemispherical shape.  
       FIG. 7  is a flowchart illustrating exemplary steps for fabricating a micro-optics device in accordance with the embodiment shown in  FIGS. 6A-6I . A core layer of an epoxy-based negative-working photo-definable polymer resist is spin-coated onto a substrate (step  700 ). If desired, the substrate with the layer of resist thereon can be thermally cured (step  710 ) as a precursor to photolithography. In the initial photolithography step (step  720 ), the core layer of resist is patterned by exposing the resist to ultraviolet (UV) light (e.g., 350 nm-400 nm) with a photo-mask having an initial pattern masking (step  730 ), subjecting the resist to a post exposure bake (step  740 ) and dissolving away unexposed regions of the resist in a developer solution (step  740 ), leaving one or more core stacks of resist material.  
      If one or more spacers of resist material are desired to widen the lens without increasing the height of the lens (step  750 ), one or more additional layers of resist can be spin-coated over the defined core stacks in the first layer of resist (step  700 ), soft-baked (step  710 ) and, to define the spacers (step  725 ), dissolved in developer solution without patterning (no UV exposure) (step  745 ). Thereafter, if additional layers of resist are to be applied (depending upon the desired height and curvature of the lens) (step  750 ), each additional layer of resist is spin-coated over the previously defined core stack and spacers of resist (step  700 ), soft-baked (step  710 ) and photolithographically patterned using a photo-mask having a larger pattern masking that is capable of producing one or more shells of resist that are larger in area than the combination of the core stack and spacers of resist and that overly one or more of the core stacks and spacers of resist (step  735 ). The resist is baked (step  740 ), and unexposed areas of resist are dissolved away in developer solution (step  740 ), leaving a stack of “shells”, where the bottom core stack has the smallest area and the top shell has the largest area.  
      A final smoothing layer of resist (step  750 ) is spin-coated over the previous patterned shells of resist (step  700 ) and soft-baked (step  710 ). The final layer is also exposed to UV light (step  735 ), subjected to a post exposure bake (step  740 ) and developed (step  740 ), such that the final patterned layer of resist covers all other layers of resist. The resulting shells of resist are thermally cured (step  760 ) to allow the final layer to flow smoothly over the other resist layers, and to allow the surface tension of the melted final layer of resist to pull the final resist layer into a lens shape having a curved surface. To finalize the lens shape and size, the lens is blanket exposed (i.e., no mask is used) with UV to cross-link the polymer material in order to harden the lens (step  770 ). A final thermal treatment can be applied, if necessary, to cure the lenses further to improve performance in subsequent processing (step  780 ).  
      The fabrication techniques described above in connection with  FIGS. 1-7  can be used to create a variety of three-dimensional optical devices with unique structures and properties. For example, referring now to  FIGS. 8 and 9 , there is illustrated an exemplary optical device  800  with refractive and diffractive properties that can be fabricated in accordance with embodiments of the present invention.  FIG. 8  is a perspective view of the exemplary optical device  800 .  FIG. 9  is a cross-sectional view of the optical device  800  of  FIG. 8 . The optical device  800  shown in  FIGS. 8 and 9  combines a large-scale curved surface-relief pattern, with a fine-detailed, microscopic diffractive pattern in order to produce a high performance, high functionality optical device with both refractive and diffractive properties.  
      More specifically, the optical device  800  shown in  FIGS. 8 and 9  includes a large-scale, curved-surface discrete relief structure  840  disposed over a substrate  830  and a fine-detailed, diffractive discrete relief structure  850  disposed over the curved-surface discrete relief structure  840 . Each of the discrete relief structures  840  and  850  are formed of an optically-transparent polymer material, such as an epoxy-based polymer material. The curved-surface discrete relief structure  840  can be fabricated as discussed above, and has a curved surface that provides refractive properties to perform the majority of the focusing and steering of light. The diffractive discrete relief structure  850  includes a surface curvature envelope  810  formed of discontinuous diffractive features  820 . The discontinuous diffractive features  820  introduce diffractive properties into the optical device  800 , such as beam splitting, grating spectroscopy, or computer generated holography.  
      Although the diffractive properties could be implemented using a pure diffractive surface, the efficiency and resulting performance of the optical device would be low. Therefore, by allowing a curved-surface discrete relief structure  840  to perform the majority of the focusing, the diffractive features  820  can be efficiently implemented and uniquely designed. In addition, the present invention is not limited to the particular shape or design of the curved-surface discrete relief structure  840  or the diffractive discrete relief structure  850  of the optical device  800  shown in  FIGS. 8 and 9 , but instead is intended to include any arbitrary three-dimensional optical device fabricated in accordance with embodiments of the present invention to provide refractive and diffractive properties. Furthermore, the optical device  800  can be fabricated in mass production using a “master molding.” 
      As can be seen in  FIG. 9 , the discontinuous diffractive features  820  include sets of fine-scale diffractive steps that create a high-frequency diffraction grating with sharp, discontinuous features. The sets collectively form the overall surface curvature envelope  810 . Each set includes a number of diffractive steps designed to provide the desired diffractive properties and to form the surface curvature envelope  810  of the optical device  800  from the top layer (top step) of each set. Thus, the surface curvature envelope  810  of the optical device  800  is produced by controlling the number of steps and the height of each step in adjacent sets of diffractive steps.  
      For example, in one embodiment, each of the sets of diffractive steps is fabricated by separately photolithographically patterning one or more layers of the optically-transparent polymer resist material. For example, the diffractive steps in each set can be fabricated as described above in connection with  FIGS. 2A-2D  or  FIGS. 4A-4C . However, the diffractive discrete relief structure is not reflowed (i.e., thermally cured) in order to maintain the sharp, discontinuous diffractive features  820 . To finalize the optical device  800 , the optical device  800  is blanket exposed (i.e., no mask is used) with UV to cross-link the polymer material in order to harden the optical device  800 .  
      In an exemplary embodiment, the diffractive discrete relief structure  850  of the optical device  800  produces an overall sag (depth) between 0-50 μm. In addition, the diffractive features  820  (diffractive steps) each have individual step heights of 0.1 to 1.5 μm, and the total stacked thickness of the optical device  800  ranges from 0.25 to 3.0 μm. Furthermore, the lateral dimension of each of the diffractive features  820  ranges from 0.4 to 10+ μm.  
      As will be recognized by those skilled in the art, the innovative concepts described in the application can be modified and varied over a wide range of applications. Accordingly, the scope of patented subject matter should not be limited to any of the specific exemplary teachings discussed, but is instead defined by the following claims.