Abstract:
A lens, such as a lens for use in a wafer-level camera, is made by forming a polymeric material with at least one master to form a pre-final lens. The pre-final lens forms a majority of a final volume of the lens. The pre-final lens is allowed to harden, during which it may sag or shrink. An aliquot of polymeric material is added to the lens and formed with the same master with a spacer, or with a second master, to form a first surface layer that provides correction between the pre-final lens shape and a final desired lens shape. In an embodiment, the surface layer has similar or identical index of refraction to the pre-final lens. In an embodiment the lens is formed on a substrate. In an embodiment, a send master, or master pair, are used to form a lens having upper and lower curvature, with a second aliquot of polymeric material forming a second surface layer on a surface of the lens opposite to the first surface layer.

Description:
TECHNICAL FIELD 
     This disclosure relates generally to optical objects that are fabricated by molding, and in particular but not exclusively, relates to wafer level optical lenses that are molded with masters. 
     BACKGROUND INFORMATION 
     Mobile phones, PDAs, laptop computers and other electronic devices often contain imaging devices such as cameras assembled as wafer level optical devices. The main components of a wafer level camera typically include a lens assembly of one or more stacked lenses, and an underlying image sensor. Wafer level cameras are manufactured by wafer level packaging technologies that include processes such as fowling opto-wafers, aligning several layers of wafers, dicing, and finally packaging individual camera modules. 
     An opto-wafer contains a multitude of small individual lenses that are fabricated onto a substrate wafer, and may be manufactured with techniques such as using a master wafer, mold, or stamp, to reproduce lenses onto a substrate wafer. Each opto-wafer typically has multiple lenses and/or spacers; these wafers are cut during dicing such that each camera module typically inherits only one lens and/or spacers from each opto-wafer. 
     SUMMARY 
     A lens, such as a lens for use in a wafer-level camera, is made by forming a polymeric material with at least one master to form a pre-final lens. The pre-final lens is at least partially cured, during which it may sag or shrink. An aliquot of polymeric material is added to the lens and formed with the same master with a spacer, or with a second master, to form a first surface layer that provides correction between the pre-final lens shape and a final desired lens shape. In an embodiment, the surface layer has similar or identical index of refraction to the pre-final lens. In an embodiment the lens is formed on a substrate. In an embodiment, a send master, or master pair, are used to form a lens having upper and lower curvature, with a second aliquot of polymeric material forming a second surface layer on a surface of the lens opposite to the first surface layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
         FIGS. 1A through 1C  are cross sectional views showing the PRIOR ART process of lenses being formed on a substrate by a hard stamp. 
         FIGS. 2A through 2C  are cross sectional views showing the process of lenses being formed on a substrate by a soft stamp. 
         FIGS. 3A through 3H  are cross sectional views showing a trial and error process wherein a mold is modified in successive rounds to match the desired specs of the final lens product. 
         FIGS. 4A and 4B  are cross sectional views showing a process wherein a lens is cast in two molding operations, each using its own master, to produce a desired final lens product. 
         FIGS. 5A through 5D  are cross sectional views showing a process wherein a lens is cast in two molding operations with essentially the same master to produce a desired final lens product. 
         FIGS. 6A and 6B  are cross sectional views showing embodiments of a lens-in-pocket form and a suspended lens. 
         FIGS. 7A and 7B  are cross sectional views showing prior art embodiments of a convex lens-in-pocket and a concave lens-in-pocket, respectively. 
         FIGS. 8A through 8C  are cross sectional views showing a process of casting a lens-in-pocket with two masters and a process of casting a lens-in-pocket with a combination of one master and one or several vertical spacers. 
         FIGS. 9A through 9C  are cross sectional views showing a process of casting a suspended lens with two sets of masters and a process of casting a suspended lens with a combination of one set of masters and one or several vertical spacers. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the following description, numerous details and alternatives are set forth to provide a thorough understanding of the present invention. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics herein described may be combined in a suitable manner in one or more embodiments. 
     Lens replication for volume production of wafer-level optical systems may be done by replication techniques such as molding. In one example, the master is a hard stamp. As shown in  FIG. 1A , a hard stamp  100 , such as one made from glass or steel, presses down upon a droplet of replication material  110 , such as an UV-curable polymer, that has been previously dispensed on a substrate wafer  120 . Replication material  110  may also be a resin cured with a catalyst, or a heated thermoplastic. As the hard stamp  100  squeezes down on the polymer, the polymer conforms to the master, as shown in  FIG. 1B , and is cured into a solid state by means such as UV light exposure, heat, or a catalyst. In  FIG. 1C , the hard stamp  100  parts from the substrate wafer  120 , leaving behind a lens layer  150  comprising individual formed lenses  130 . If an excess of replication material  110  is provided, a residual layer  140  may also be present beneath the formed lenses. In another example, the master is a soft stamp.  FIG. 2A  shows a soft stamp  200  that may be made from silicone rubbers such as polydimethylsiloxane (“PDMS”). Replication material  210 , such as UV epoxy polymer, is dispensed onto the soft stamp  210 .  FIG. 2B  shows the soft stamp  210  pressing down upon the substrate wafer  120 , transferring the polymer  210  onto the substrate wafer  120 , and casting the polymer  210  into lens. The polymer  210  is cured into a solid state by means such as UV exposure. Parting of the soft stamp  200  from the substrate wafer  120  leaves behind a lens layer  230  having multiple individual lenses  220 , as shown in  FIG. 2C . 
     After a lens is molded, it may experience shape change while its polymer cures, an example of which is shrinkage or sagging. Shrinkage is related to curing properties of the lens material, such as epoxy. Shape change such as shrinkage results in a lens product that deviates from the shape of the stamp, and which may deviate from intended lens design specifications. 
     Several methods may be used to remedy the undesirable shrinkage effect. In one approach, shrinkage is compensated for by using a mold, such as hard stamp  100  that has a shrinkage-compensated shape to produce a pre-shrunk, oversized, lens that is slightly larger than the desired goal. The hope is that as the oversized lens shrinks to a smaller size, the final form will meet the intended lens design specifications. This approach is a trial-and-error process that is often cumbersome because, among other reasons, shrinkage is sometimes not exactly as predicted and may vary with temperature and precise material composition. 
       FIGS. 3A through 3H  illustrate an example of this prior-art process. A first trial master  300  is made to include a cavity that matches shape of the intended lens. The first trial master  300  is used to cast a polymer  310  onto a substrate  120 , as shown in  FIG. 3A , to form a first trial lens  320 , as shown in  FIG. 3B . As the first trial lens  320  solidifies, cures, and stabilizes, it shrinks from its initially molded dimensions  332  to a first smaller lens  330 , as shown in  FIG. 3C . Comparing the first smaller lens  330  with intended lens specifications enables an engineer to estimate the extent to which the cavity of the first trial master  300  needs to be enlarged in order to produce a second trial master  305 , as shown in  FIG. 3D ; the first trial lens is then discarded. Then, in a separate molding operation, the second trial master  305  is used to cast a larger amount of polymer  315  onto the substrate  120 , as shown in  FIG. 3E , thereby forming a second trial lens  325 , as shown in  FIG. 3F . The second trial lens  325  eventually shrinks from its initially molded dimensions  334  to a second smaller lens  335 , as shown in  FIG. 3G , during curing if it is formed from a UV cured polymer or a catalytically cured epoxy polymer, or during cooling if it is formed from a thermoplastic. Hopefully this second smaller lens  335  is sufficiently close to the intended lens specifications. If not, then this iterative trial-and-error process will continue, until a final lens product  340  that sufficiently resembles the intended lens shape is produced, as shown in  FIG. 3F . Two, three, or more rounds of mold machining are often required to eventually produce a final master that can be used to mold a lens that meets the lens design specifications. 
     We propose a process that allows avoidance of the trial-and-error, iterative, process, while producing a final lens-on-flat-substrate by using two or more successive and additive molding operations. In one embodiment, two masters are used to cast a lens on a flat substrate. In a first molding operation, the first master, alternatively known as the blob mold, and as illustrated in  FIG. 4A , is used to cast onto substrate  120  a first lens, which is then at least partially cured, during curing it is allowed to shrink. After shrinking, the first lens becomes a pre-final lens  410 , alternatively known as a blob  410 . The amount of lens material that is used for the blob  410  typically constitutes most, but not all, of total material that is expected to be used for the final lens. For example, in an embodiment, approximately 95% to 99% of the intended total of lens material is used to cast the blob  410 . Once cast, the blob is cured sufficiently that most or all shrinkage the blob will undergo occurs. Since the blob contains most of the total material of the final lens, most of the bulk or large scale shrinkage effects of the blob have happened in the blob  410  when this curing step is complete. 
     The blob  410  may closely approximate the intended design specs of the desired final lens. For example, the blob  410  may have dimensional measurements that are up to 10 micrometers smaller than those of the desired final lens. 
     Following shrinkage of the blob  410 , the second master is used in a second molding operation to transfer a thin layer of additional polymer  420  onto the blob  410 , as shown in  FIG. 4B . The second master, alternatively known as the lens mold, may have the exact lens shape as the desired final lens, or in an alternative embodiment may include a small modification for shrinkage compensation vis-à-vis the desired final lens. The amount of additional material transferred onto the blob  410  during the second molding operation may be, for example, approximately 1% to 5% of the total amount of lens material, the sum of blob material and additional material being approximately 100% of the lens material of the finished lens. Due to the small amount of lens material that is used to cast the thin layer  420 , this thin layer  420  only experiences slight shrinkage relative to final lens shape. While the material of thin layer  420  may be the same or a similar material to that of the blob, because shrinkage in a particular dimension is proportional to the amount of material present and thin layer  420  is much thinner than the blob, shrinkage of the thin layer represents a smaller change in overall lens dimensions than would shrinkage of an entire lens. The shrinkage effects of thin layer  420  on final shape of the thin layer  420  are therefore significantly reduced from those of the blob, and the thin layer backfills and compensates for shrinkage of the blob. As a result, the final lens  430 , comprising the blob  410  and the thin layer  420 , should be sufficiently close to the designed shape of the desired final lens shape to meet lens design specifications. 
     Due to the several separate and additive molding operations, an interface may exist between the blob  410  and the thin layer  420 . To prevent distortion, reflections, or scattering from such an interface, the blob  410  and the thin layer  420  may be made of the same lens material, or a material with the same or similar optical indices including the index of refraction as the blob  410 . In an alternative embodiment thin layer material  420  has lower viscosity than blob material but the same or similar index of refraction to enable easy molding of the thin layer  420 . In an embodiment, thin layer material  420  is the same material as blob  410  and has the same index of refraction, in another embodiment it has a difference in index of refraction from blob  410  within 5% of that of blob  410 , and in an alternative embodiment a difference in index of refraction within 10%. In an embodiment, thin layer  420  adheres directly to blob  410  with no need for an interfacial layer, and has an index of refraction sufficiently close to that of the blob  410  that essentially no refraction occurs at a boundary  422  of thin layer  420  to blob  410 . In alternative embodiments, an interfacial substance such as an index matching and bonding material may be dispensed at the potential interface, at the boundary  422  between thin layer  420  and blob  410 . In addition or as an alternative, following the casting of the thin layer  420 , additional process steps may be employed so as to mitigate any possible interface effect. For example, a reflow step may be used to diminish the interface between the blob  410  and the thin layer  420  by blending the materials of blob and thin layer at the interface, such that some of thin layer  420  merges into and thickens blob  410 . 
     In certain situations, such as when a PDMS soft stamp is used for casting, a lens cast on a flat substrate may experience shrinkage primarily in the vertical direction. In addition, the compressible, rubber-like, nature of PDMS may also contribute to shrinkage and deformation. In another embodiment wherein two or more successive, additive, molding operations are used to produce a lens-on-flat-substrate, a first master  500 , as shown in  FIG. 5A , is used to cast a pre-final lens or blob  510  onto a substrate  120 , as shown in  FIG. 5B . A second master  550  is constituted by adding one or several spacers  540  to the first master  500 , as shown in  FIG. 5C . The second master  550  is used to transfer a thin layer  520  onto the blob  510 , as shown in  FIG. 5D . In many embodiments, the final lens shape  530  of blob  510  plus thin layer  520  is sufficiently close to the intended design specs of the desired final lens shape as to need no further compensation; if correction is required the magnitude of subsequent master revisions is much reduced from that required with the prior lensmaking procedures. 
     Alternative lens forms exist in addition to the lens-on-flat-substrate form as disclosed above. For example, in an alternative embodiment, a lens having a lens-in-pocket form, such as where spacers  600 , positioned between lenses  610 , are situated on a substrate  120 , and as shown in  FIG. 6A  may be fabricated. In another example, a suspended lens  620  may be fabricated, such as where lens  620  is positioned between and elevated with respect to spacers  600 , as shown in  FIG. 6B . 
     Lens-in-pocket or suspended lenses may present additional challenges to control lens deformation because of forces like lateral stresses from the side wall of the spacers, as illustrated in  FIG. 7A . Master  720  is used to cast a convex lens-in-pocket form  710  onto a substrate  120 , where convex lens-in-pocket form  710  is bounded by spacers  700 . Voids  730  serve as overflow space that permits the side  740  of the lens to overflow. Forces such as the lateral stress  750  may make it difficult to predict shrinkage compensation, and impair process repeatability when shrinkage-compensated molds, such as that of  FIG. 3E , are used. For example, in addition to a vertical deformity or shrinkage, the lateral stress  750  may induce a lateral deformity or shrinkage, which may be restrained by spacers  700  thereby causing further lens deformations. 
     In addition to the effect of lateral stress from spacers  700 , the extent to which the overflowed lens material at the lens side  740  bulges into the voids  730  may exacerbate the lateral stress  750 . An example that illustrates this phenomenon is shown in  FIG. 7B . Here, a master  725  is used to cast onto a substrate  120  a concave lens-in-pocket form  715  which is bounded by spacers  700 . Voids  735  at periphery of master  725  serve as overflow space that permits the side  745  of the lens to overflow as well as allowing for minor misalignments of master  725  to substrate  120 . Forces such as lateral stress  755  may make it difficult to control shrinkage compensation. A concave embodiment may have more pronounced lateral stress effects than many convex embodiments. In part due to gravity, the overflowed lens material at the lens side  745  may magnify the lateral stress  755 , and may potentially cause significant lateral deformity or shrinkage. Further, if there are variations in the amount of the lens material such that there is a variation in the amount of the overflowed lens material  745 , then the level of difficulty of compensating for shrinkage may be increase, in part due to possible variations in vertical and/or lateral deformations or shrinkages. 
     Several embodiments of our process for forming a lens-in-pocket form are disclosed herein. In one embodiment, as shown in  FIG. 8A , a first master, such as a soft PDMS master  820 , is used to cast a pre-final lens  810  onto a substrate  120 , with the pre-final lens  810  being bounded by spacers  800 . After the pre-final lens  810  has experienced shrinkage and hardened, a second master, such as a soft PDMS master  825 , is used to transfer a small amount of lens material onto the pre-final lens  810 , thereby forming a thin layer  815 , as shown in  FIG. 8B . In another embodiment, after the pre-final lens  810  has been casted by the master  820  as shown in  FIG. 8A , vertical spacers or shims  805  ( FIG. 8C ) are added to spacers  800 . Then the same master  820  is re-used with some additional lens material to form a thin layer  815  onto the pre-final lens  810 , as shown in  FIG. 8C ; the vertical spacers or shims  805  operate to control thickness of the thin upper layer  815 . 
     Structural and operational parameters for the lens-in-pocket form are similar to those of the lens-on-flat-substrate form. For example, the pre-final lens  810  may constitute approximately 95% to 99% of the entire amount of lens material; since shrinkage is typically proportional to volume most of the shrinkage or deformation effect occurs prior to adding thin layer  815 . In another example, the pre-final lens  810  may have dimensional measurements that are up to ten micrometers less than those of the desired final lens shape, thereby closely approximating the desired final lens shape. Yet in another example, an interface may exist between the pre-final lens  810  and the thin layer  815 , and the interface may produce surface related errors, such as light distortion, refraction and reflection at the interface. Accordingly, the pre-final lens  810  and the thin layer  815  may be made from the same lens material, or a different material with the same or similar optical refractive index, in order to reduce or eliminate reflection and refraction at the interface. Further, an interfacial substance such as an index matching material may be dispensed at the potential interface. Following the casting of the thin layer  815 , an additional process step, such as a reflow step, may be employed so as to mitigate interface effects by blending the thin layer  815  into blob  810 . 
     A suspended lens, similar to the lens-in-pocket form, is bounded by spacers. On the other hand, for a suspended lens, both the upper surface and the lower surface need molds while the lens is casted. Several embodiments regarding the suspended lens are disclosed herein. In one embodiment, as shown in  FIG. 9A , a first upper master  922  and a first lower master  924  are used to cast a pre-final lens  910  while being bounded by spacers  900 . After the pre-final lens  910  has experienced shrinkage, a second upper master  926  ( FIG. 9B ) and a second lower master  928  may each be used to transfer a small amount of lens material onto the upper surface and the lower surface of the pre-final lens  910 , thereby forming a thin upper layer  912  and a thin lower layer  914 . In another embodiment, after the pre-final lens  910  has been casted by the upper master  922  and the lower master  924 , as shown in  FIG. 9A , vertical spacers or shims  905  ( FIG. 9C ) are added to top and bottom of the spacers  900 . Then the same upper master  922  and the same lower master  924  as used for initial molding of the pre-final form are used to form the thin upper layer  912  and the thin lower layer  914  onto the pre-final lens  910 , as shown in  FIG. 9C ; the vertical spacers  905  operate to control thickness of the thin upper layer  912  and the thin lower layer  914 . 
     Structural and operational parameters for suspended lenses are similar to those of the lens-in-pocket form. For example, the pre-final lens  910  may constitute approximately 95% to 99% of the entire amount of lens material, thereby most of the shrinkage or deformation effect is in the pre-final lens. In another example, the pre-final lens  910  may include dimensional measurements that are up to 10 micrometers less than those of the desired final lens shape, thereby closely approximating the desired final lens shape while allowing for additional material. 
     Yet in another example, an interface may exist between the pre-final lens  910  and the upper and low thin layers  912  and  914 , and this interface may produce surface related errors, such as light distortion, as previously discussed with reference to the lens-in-pocket form. Interface effects can be reduced or eliminated by using the same lens material, or material with the same or similar optical indexes, for both the pre-final lens  910  and the thin upper and lower layers  912  and  914 . Further, an interfacial substance such as an index matching material may be dispensed at the potential interface. In addition or as an alternative, following the casting of the thin upper and lower layers  912  and  914 , a reflow or other process step may be employed so as to mitigate the possible interface effect by blending thin upper and lower layers  912 ,  914  with material of the pre-final lens  910  at the interface. 
     It should be noted that the method of forming lenses herein described is adaptable to forming lenses of concave, convex, spherical, or aspherical form; the shape of the resulting lenses is determined by shape of the masters used for forming the lenses. 
     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.