Patent Publication Number: US-2022227080-A1

Title: Replicating optical elements onto a substrate

Description:
TECHNICAL FIELD 
     This disclosure relates to replicating optical elements. 
     BACKGROUND 
     Optical devices that include one or more optical light emitters and one or more optical sensors can be used in a wide range of applications including, for example, distance measurement, proximity sensing, gesture sensing, and imaging. Small optoelectronic modules such as imaging devices and light projectors employ optical assemblies that include lenses or other optical elements stacked along the device&#39;s optical axis to achieve desired optical performance. Replicated optical elements include transparent diffractive and/or refractive optical elements for influencing an optical beam. In some applications, such optoelectronic modules can be integrated into various consumer electronics, such as portable computing devices (e.g., smart phones, tablets, wearables, and laptop computers). 
     SUMMARY 
     The present disclosure describes techniques for controlling the flow of replication material (e.g., epoxy) during the formation of replicated optical elements. The techniques involve providing a transparent (e.g., glass) substrate onto which the optical elements are to be replicated. The substrate includes a structured UV curable shield adhering to its surface. The UV curable shield, in turn, has openings that expose portions of the surface of the transparent substrate for replication of the optical elements. During the replication process, excess replication material may flow onto the UV curable shield, which subsequently can be cured so as to facilitate the release and removal of the shield along with the excess replication material. 
     In various implementations, the replication tool includes spacers to facilitate the process. 
     For example, in one aspect, the present disclosure describes a method that includes providing a transparent substrate having a structured UV curable shield adhering to its surface. The UV curable shield has openings that expose portions of the surface of the transparent substrate. The method further includes replicating optical elements onto the exposed portions of the surface of the transparent substrate using a replication tool having a replication material on respective replication surfaces corresponding to optical elements. The replication tool further includes spacers each of which laterally surrounds a respective one of the replication surfaces, and wherein, during the replicating, a free-end of each spacer is brought into close proximity to an opposing surface of the UV curable shield. UV radiation is applied to the replication material and to the UV curable shield, and subsequently the shield is removed from the transparent substrate. 
     Some implementations include one or more of the following features. For example, in some instances, the UV curable shield is composed of a dicing tape. Applying the UV radiation can release the shield from the transparent substrate. In some implementations, during the replicating, excess replication material flows onto the UV curable shield, and removing the shield also removes the excess replication material. In some cases, during the replicating, the free-ends of at least some of the spacers are brought into contact with the opposing surface of the UV curable shield. In some instances, during the replicating, when the free-ends of the spacers are brought into close proximity to the opposing surface of the UV curable shield, a sub-micron thick layer of the replication material is present between at least some of the free-ends of the spacers and the opposing surface of the UV curable shield. The transparent substrate can be composed, for example, of glass. 
     In accordance with another aspect, the present disclosure describes a method that includes providing a transparent substrate having a structured UV curable shield adhering to its surface. The UV curable shield has openings that expose portions of the surface of the transparent substrate. The method further includes replicating optical elements onto the exposed portions of the surface of the transparent substrate using a replication tool having a replication material on respective replication surfaces corresponding to optical elements. The replication tool further includes spacers each of which laterally surrounds a respective one of the replication surfaces, and during the replicating, a free-end of each spacer is brought into close proximity to a respective one of the exposed portions of the surface of the transparent substrate. UV radiation is applied to the replication material and to the UV curable shield, and subsequently the shield is removed from the transparent substrate. 
     Some implementations include one or more of the following features. For example, in some instances, the UV curable shield is composed of a dicing tape. Applying the UV radiation can release the shield from the transparent substrate. In some instances, during the replicating, excess replication material flows onto the UV curable shield, and removing the shield also removes the excess replication material. In some cases, during the replicating, the free-ends of at least some of the spacers are brought into contact with the respective one of the exposed portions of the transparent substrate. In some implementations, during the replicating, when the free-ends of the spacers are brought into close proximity to a respective one of the exposed portions of the transparent substrate, a sub-micron thick layer of the replication material is present between at least some of the free-ends of the spacers and the respective one of the exposed portions of the transparent substrate. The transparent substrate can be composed, for example, of glass. 
     According to yet a further aspect, the present disclosure describes a method that includes providing a transparent substrate having a structured UV curable shield adhering to its surface. The UV curable shield has first and second openings that expose portions of the surface of the transparent substrate, wherein respective groups of the second openings laterally encircle respective ones of the first openings. The method includes replicating optical elements onto the exposed portions of the surface of the transparent substrate defined by the first openings. The replicating is performed using a replication tool having a replication material on respective replication surfaces corresponding to optical elements, wherein the replication tool further includes a plurality of spacers, wherein respective groups of the spacers laterally encircle respective ones of the replication surfaces. During the replicating, a free-end of each spacer is brought into contact with a respective one of the exposed portions of the surface of the transparent substrate defined by the second openings. UV radiation is applied to the replication material and to the UV curable shield, and subsequently the shield is removed from the transparent substrate. 
     Some implementations include one or more of the following features. For example, in some instances, the UV curable shield is composed of a dicing tape. Applying the UV radiation can release the shield from the transparent substrate. In some cases, during the replicating, excess replication material flows onto the UV curable shield, and removing the shield also removes the excess replication material. The transparent substrate can be composed, for example, of glass. 
     Some implementations provide one or of the following advantages. For example, in some cases, the techniques help control the flow of replication material over the surface of the substrate. Controlling the flow of the replication material can, in some instances, help reduce the overall footprint of each optical element, which in turn can help reduce the size of the package or module into which the optical element is integrated. 
     Other aspects, features, advantages will be apparent from the detailed description, the accompanying drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross-sectional view of a tool-substrate structure for replication. 
         FIG. 2  shows a replicated optical element having an excess epoxy or “yard” portion. 
         FIGS. 3A, 4, 5A, 6 and 7A  show a sequence of steps in a first process for fabricating replicated optical elements. 
         FIG. 3B  is an enlarged view of section A of  FIG. 3A . 
         FIG. 5B  is an enlarged view of section B of  FIG. 5A . 
         FIG. 7B  is an enlarged view of section C of  FIG. 7A . 
         FIG. 8A  illustrates a second implementation. 
         FIG. 8B  is a top view of a slice through A-A in  FIG. 8A . 
         FIG. 9A  illustrates a third implementation. 
         FIG. 9B  is a top view of a slice through A-A in  FIG. 9A . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows a cross section of a typical replication tool  101 , and a transparent substrate  120  onto which optical elements are to be formed by replication. The tool  101  includes a rigid or relatively hard back plate  102  composed of a first material, for example glass, and a replication portion  104  composed of a second, softer material, for example polydimethylsiloxane (PDMS). The relatively low stiffness of the replication portion  104  can allow the replication portion, under “normal” conditions (e.g., where no more pressure than the one caused by gravity forces of the tool lying on the substrate or vice-versa), to adapt to roughness, e.g., on a micrometer and/or sub-micrometer scale and, thus, may form an intimate connection to the substrate surface when they are brought into contact with one another. 
     The replication portion  104  forms a replication surface  108  including replication sections  106 , the surface of each of which is a (negative) copy of a surface shape an optical element to be manufactured by replication. The optical elements to be manufactured by replication may be, for example, lenses, diffusers, or other optical elements. In some instances, each optical element to be replicated is a microlens array (MLA). In some cases, the replication sections  106  can be, for example, convex and thus define a concave optical element surface, or can be convex and define a concave optical element surface. 
     The replication portion  104  has contact spacer portions  112  arranged peripherally. The contact spacer portions  112  are the structures of the replication tool  101  that protrude the furthest from the tool  101  along the z axis. The contact spacer portions  112  are essentially flat and, thus, are operable to rest against the substrate  120  during replication, with no material between the contact spacer portions  112  and the substrate  120 . The contact spacer portions  112  may, for example, form a ring laterally surrounding the periphery of the replication surface  108 , or may form discrete portions around the periphery. 
     The substrate  120  has a first side (e.g., substrate surface  126 ) and a second side and can be composed of any suitable material, for example glass. The substrate surface  126  may have a structure to which the replica is to be aligned. The structure may, for example, comprise a coating  122  structured in the x-y-plane, such as a screen with apertures, or a structured IR filter etc. The structure may in addition, or as an alternative, comprise further features like markings. 
     For replicating the replication surface  108  of the tool  101 , replication material  124  is applied to the substrate  120  or the tool  101  or both the tool  101  and the substrate  120 . Although a single portion of replication material  124  is illustrated in the figure, application of the replication material  124  may include applying multiple portions of replication material  124  (e.g., a respective portion for each of the replication sections  106 ). Each portion may, for example, be applied by dispensing (e.g., jetting) one or more droplets using a dispensing tool. The replication material  124  can be composed, for example, of epoxy. 
     After application of the replication material  124 , the substrate  120  and the tool  101  are aligned with respect to one another, for example, at an alignment station. Subsequent to the alignment, the substrate  120  and the tool  101  are brought together, with the contact spacer portions  112  resting against the substrate surface so as to define the height in the z dimension and also to lock the tool against x-y-movements. After the replication tool  101  and the substrate  120  have been moved towards each other with the replication material  124  between them, the substrate-tool-assembly can be removed from the alignment station and transferred to a hardening station, where the replication material  124  is hardened (e.g., cured). The replication tool  101  then can be removed. 
     Referring to  FIG. 2 , during replication, excess replication material (e.g., epoxy) applied, for example, during jetting normally overflows the region of interest and forms a “yard”  130  when the tool and the substrate  120  are brought into contact. The yard  130  sometimes is annular or ring shaped and laterally surrounds the active region of the optical element  131 . The yard  130  results from more epoxy  124  being added during the replication process than each replicated structure (e.g., optical element) requires, causing an overflow. The additional epoxy ensures that the complete volume of replication material needed for a particular structure is available (as the tolerance of the epoxy volume is not zero), and the extra fluid pools to form the yard  130 . 
     The transparent substrate having the replicated optical elements on its surface then can be separated into individual units each of which includes a single one of the replicated optical elements (e.g., MLAs). The replicated optical elements then can be positioned (e.g., by pick-and-place equipment), for example, over a light emitter such as a VCSEL, an LED or laser diode as part of an optoelectronic package that subsequently may be assembled into a small portable computing device such as a smartphone. Space in such devices, however, is often at a premium. Thus, it is desirable in many instances to reduce the footprint or area covered by the optoelectronic package, which in turn can impose tight requirements on the maximum dimensions of the optical element unit. Thus, it is desirable to reduce the footprint of the optical element unit. 
       FIGS. 3A through 7B  illustrate a first process for fabricating replicated optical elements, which can result in the optical elements having a reduced overall footprint even though the active region of the optical element has substantially the same dimensions as the optical elements resulting from the process of  FIGS. 1-2 . The replicated optical elements can be fabricated in a wafer-level process in which tens, hundreds or even more optical elements are fabricated in parallel. 
     As shown in  FIGS. 3A and 3B , a thin, structured, ultra-violet (UV) curable shield  202  is provided on a surface of a transparent (e.g., glass) substrate  220 . Prior to placing the UV curable shield  202  on the substrate  220 , the surface of the substrate can be prepared, for example, by plasma activation, which can help improve adherence of the UV curable shield  202  to the substrate. The UV curable shield  202  can be, for example, dicing tape that is structured, e.g., by forming openings  203  in the dicing tape using water jetting. In some instances, the UV curable shield is placed on the substrate  220  using a mask aligner. The openings  203  in the UV curable shield  202  correspond to locations on the substrate  220  where the replicated optical elements are to be formed. In some instances, dicing tape having a thickness of about 75 μm can be used. The dicing tape can by a one-sided adhesive tape in which the side having the adhesive faces the transparent substrate  220 . 
     Next, as illustrated in  FIG. 4 , replication material (e.g., a polymer such as an epoxy) is provided (e.g., by jetting) on the replication surface  208  of a replication tool  201 . As described above, the tool  201  can include a rigid or relatively hard back plate composed of a first material, for example glass, and a replication portion composed of a second, softer material, for example PDMS. The replication material  124  is provided on optical element replication regions  206  of the replication surface  208 . The optical element replication regions  206  define the (inverse) of the optical elements to be replicated onto the surface of the substrate  220 . In this implementation, however, the replication portion of the tool  201  also has spacers  210 , each of which laterally surrounds a respective one of the optical element replication regions  206 . The spacers  210  extend beyond areas of the replication surface  208  that define the optical element replication regions  206 . 
     After application of the replication material  124 , the substrate  220  and the tool  201  are aligned with respect to one another, for example, at an alignment station. Subsequent to the alignment, the substrate  220  and the tool  201  are brought together (see  FIGS. 5A and 5B ). As described in connection with  FIG. 1 , the tool  201  may have contact spacer portions at its outer periphery that rest against the substrate surface so as to define the height in the z dimension and also to lock the tool against x-y-movements. In addition, however, the spacers  210  that laterally surround individual ones of the optical element replication regions  206  are designed so that, when the tool  201  is brought into contact with the substrate  220 , the free-end of each spacer  210  contacts the opposing surface of the UV curable shield  202  (see  FIGS. 5A and 5B ). As the tool  201  is brought into contact with the substrate  220 , excess replication material  124 A is squeezed out of the regions defining the replicated optical elements. As a result, the excess replication material  124 A is disposed on the UV curable shield  202 . In some cases, a very thin (e.g., sub-micron) layer of replication material  124  may remain between the free-end of the spacer  210  and the opposing surface of the UV curable shield  202 . 
     After the optical elements are replicated on the transparent substrate  220  as described above, the substrate-tool-assembly can be transferred to a hardening station, where a UV curing process is applied. The UV curing process cures the replication material  124  (including the excess replication material  124 A) and also releases the UV curable shield  202  so that it can be detached from the substrate  220  in a subsequent operation. 
     The replication tool  201  then can be removed. The resulting structure is illustrated in  FIG. 6 , which shows the cured replication material  124  for the optical elements disposed directly on the transparent substrate  220 , and also shows the cured excess replication material  124 A disposed on the cured shield  202 . Next, as illustrated by  FIG. 7 , the shield  202  (which has been released as a result of the UV curing), can be removed manually from the transparent substrate  220 . As the developed layer of replication material between the shield  202  and the free-ends of the spacers  210  is very thin, a clean stress break along the edges of the shield  202  can be made, resulting in optical elements (e.g., lenses) with only a very small “yard”  230  and a relatively clean glass surface  232  where the shield  202  previously was present. 
     In the foregoing example, the thickness of the shield  202  impacts the thickness of the base layer  234  of the replicated optical element  200  because the spacers  210  rest on opposing surface of the shield  202  during the replication process. Variations in the thickness of the shield  202  from process to process can result in variations in the thickness of the base layers of the replicated optical elements. To reduce such variations, which may impact optical performance of the replicated optical elements, the process can be modified so that the free-ends of the spacers  210  lie on the transparent substrate  220  (rather than on the UV curable shield  202 ), while most of the excess replication material  124 A still is on the UV curable shield  202 . For example, the openings in the UV curable shield can be made slightly larger than the diameter of the optical element&#39;s base so as to accommodate room for the spacer  210 .  FIGS. 8A and 8B  illustrate an example in which an opening  250  in the UV curable shield  202  is large enough to accommodate an area  252  for the replicated optical element as well as an area for the spacer  210  of the tool  201 A. As before, application of the UV radiation cures the replication material  124  (including the excess material  124 A) and also releases the shield  202 . The shield  202  then can be removed manually from the transparent substrate  220 , resulting in optical element (e.g., a lens) with only a very small “yard” and a relatively clean glass surface where the shield  202  previously was present. 
     In the example of  FIGS. 8A-8B , a very thin (e.g., sub-micron) layer of replication material  124  still may remain between the free-end of the spacer  210  and the opposing surface of the UV curable shield  202 . To achieve even greater uniformity in the thickness of the base layer of the replicated optical elements, in some instances, as shown in  FIGS. 9A and 9B , additional openings  262  can be formed in the UV curable shield  202 . The openings  262  are disposed laterally around the perimeter of the opening  250  that accommodates the area  252  for the replicated optical element and for the spacer  210  of the tool  201 B. Each of the openings  262  can be shaped and sized, for example, so that a respective spacer  260  of the tool  201 B can rest directly on the surface of the transparent substrate  220 . In this implementation, the spacers  260  are outside the area where the replicated optical element is formed. When the tool  201 B is brought into contact with the transparent substrate  220 , the spacers  260  are in direct contact with the surface of the transparent substrate and thus define the base layer thickness for the optical elements. In this process, the any excess replication material  124  flows outside and over the UV curable shield  202 . As the shield  202  subsequently is removed, it is unnecessary to control the volume of replication material very precisely. Once the replication material  124 ,  124 A is UV-cured and the shield  202  is released, removal of the shield  202  induces stress in thin regions around the replicated optical element, causing the excess replication material to be break off for removal. 
     The foregoing techniques can be performed, for example, at the wafer-level. The sub-assembly, including the transparent substrate having the replicated optical elements on its surface, then can be attached, for example, to another substrate (e.g., a printed circuit board) on which are mounted multiple light emitting devices (e.g., VCSELs, laser diodes, or LEDs). Each of the optical elements is aligned to an optical axis of a respective one of the light emitting devices. The stack of substrates then can be separated (e.g., by dicing) to form individual modules or packages each of which includes a light emitting device and an optical element. In this context, the substrate is “transparent” in the sense that it is substantially transparent to a wavelength of radiation (e.g., visible, infra-red (IR) or ultra-violet (UV)) emitted by the light emitting device. 
     In some implementations, the transparent substrate having the replicated optical elements on its surface is separated into individual units each of which includes a single one of the replicated optical elements (e.g., MLAs). The replicated optical elements then can be positioned (e.g., by pick-and-place equipment), for example, over a light emitter such as a VCSEL, an LED or laser diode as part of an optoelectronic package. 
     In some instances, a sub-assembly, including the transparent substrate having the replicated optical elements on its surface, is attached, for example, to another substrate (e.g., a printed circuit board) on which are mounted multiple light (e.g., visible, IR or UV) sensors. In this context, the substrate is “transparent” in the sense that it is substantially transparent to a wavelength of radiation (e.g., visible, infra-red (IR) or ultra-violet (UV)) detectable by the light sensor. 
     Other implementations are within the scope of the claims.