Abstract:
Methods and apparatus providing a master wafer having standoffs to control a bondline thickness between one wafer produced indirectly from the master wafer and another wafer bonded to the produced wafer. Standoffs may be formed as a single standoff, standoff walls, or as a number of discrete standoffs. The standoffs provide a balanced support base and a uniform spacing between the one wafer and another wafer bonded thereto.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to the field of wafer level package formation, and more specifically to methods used in attaching a wafer containing a lens to a wafer containing an imager die. 
       BACKGROUND 
       [0002]    A common technique of forming imaging device lenses uses a master wafer having a plurality of lens shapes formed on a surface to form an intermediate negative sub-master wafer, and subsequently uses the sub-master wafer as a lens stamp to form a plurality of lenses across a lens wafer. The lens wafer is often made of glass or other transparent substrate and is covered by a layer of lens material, for example, an acrylic polymer or other optical polymer. By pressing the sub-master wafer into the lens material, the lens shapes of the master wafer are replicated across the surface of the lens wafer. After removing the sub-master wafer, the lens wafer will have lens shapes formed in the lens material forming lens structures at locations across the wafer. The lens wafer may then be stacked atop or otherwise attached to an imager wafer containing imager dies provided at locations across the imager wafer. The imager wafer and lens wafer are aligned such that a lens is structurally positioned directly above each imager die. In order to set the proper focal length of the lens wafer, a spacer wafer may be placed between the lens wafer and the imager wafer. The thickness of the spacer wafer determines the distance between the lens wafer and the imager wafer. The joined lens wafer, spacer wafer, and imager wafer may further include additional attached wafer layers, such as additional lens wafers or filters. After all desired wafer level elements are attached together, the wafer assembly is subsequently cut to form individual imager modules for use in cameras and other imaging devices. 
         [0003]    In the course of mass manufacturing lenses in the above described manner, the vertical proximity of the lens wafer to the conjoined imager wafer should be ideally uniform in order to preserve consistency of lens structure focal lengths among the imager modules. In order to achieve this, various checks and controls may be instituted at various processing steps. 
         [0004]    Two particular variables that must be controlled are the horizontal alignment of one wafer with another and the vertical spacing between one wafer and another. For example, to ensure a consistent focal point positioning for all lens structures positioned in respective imager modules, the lens wafer needs to be consistently spaced the same amount from the imager die wafer across the entirety of both wafers. Tilting of one wafer with respect to the other will result in an undesired deviation in focal distance across the imager modules. 
         [0005]    A spacer wafer, as previously mentioned, is one way to set the focal lengths between the imager wafer and the lens wafer. The spacer wafer is often attached to the lens wafer using an adhesive, for example, an epoxy layer. A problem arises in ensuring that the epoxy bondline is uniform in thickness. If there is a different thickness in the epoxy bondline, the lens wafer which is attached to the imager wafer through the spacer wafer may not be in precise horizontal alignment with the adjoining image wafer. Spacer beads of uniform diameter can be mixed in the epoxy to control the epoxy bondline thickness, however, spacer beads can be expensive to manufacture, are limited in range of size, and are fragile. If too much pressure is exerted in pressing the spacer wafer against the lens wafer during attachment, the spacer beads may break. A more customizable, durable and less costly way of controlling the epoxy bondline thickness in wafer level fabrication is desirable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  shows a master wafer having standoffs and lens shapes formed therein. 
           [0007]      FIG. 2  shows a sub-master wafer having standoff dies and lens dies formed therein. 
           [0008]      FIG. 3  shows a cross-sectional side view of a sub-master wafer being pressed into a lens material deposited on a surface of a lens wafer. 
           [0009]      FIG. 4  shows a cross-sectional side view of the lens wafer of  FIG. 3  having standoffs and lenses formed thereon. 
           [0010]      FIG. 5A  shows a master wafer having a standoff wall formed therein. 
           [0011]      FIG. 5B  shows a sub-master wafer formed from the master wafer of  FIG. 5A . 
           [0012]      FIG. 5C  shows a master wafer having another configuration of a standoff wall formed therein. 
           [0013]      FIG. 5D  shows a master wafer having yet another configuration of a standoff wall formed therein. 
           [0014]      FIG. 6  shows a top view of a master wafer having standoffs formed therein positioned as alignment marks. 
           [0015]      FIG. 7  shows a top view of a master wafer having a plurality of standoffs populating a surface of the master wafer positioned in columns which alternate with columns of lenses. 
           [0016]      FIG. 8  shows a cross-sectional side view of a lens wafer having standoffs formed thereon with an adhesive deposited over the standoffs. 
           [0017]      FIG. 9  shows a cross-sectional side view of the lens wafer of  FIG. 8  having a spacer wafer attached to the standoffs. 
           [0018]      FIG. 10  shows a cross sectional side view of a lens wafer having standoffs formed on both sides. 
           [0019]      FIG. 11  shows a cross sectional side view of an imager module having wafer level optics stack incorporating standoffs. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and which illustrate specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to make and use them. It is also understood that structural, logical, or procedural changes may be made to the specific embodiments disclosed herein. 
         [0021]    In one embodiment, standoffs are used control the epoxy bondline thickness between a spacer wafer and an adjoining lens wafer. The standoffs may be formed in various ways including, but not limited to, being integrally formed in a lens master wafer which is used to create a negative sub-master, which in turn is used to produce a lens wafer having standoffs. Furthermore, standoffs may be formed in various shapes, sizes, patterns and layouts. 
         [0022]    Referring now to the drawings, where like elements are designated by like reference numerals,  FIG. 1  shows an embodiment of a master wafer  4  having integral standoffs  8  and lens shapes  5  formed on a surface of a substrate  6 . The substrate  6  may comprise a wafer of silicon, metal, or any other suitable material for use in wafer level fabrication processes. The standoffs  8  may be formed on the substrate  6  through the same conventional techniques used to form lens shapes  5  on the master wafer  4 , and using the same material used to form the lens shapes. Such techniques, including photolithography, etching and other methods, are well known in the art and will not be discussed further here. 
         [0023]    The standoffs  8  may be created having a uniform height for controling an adhesive bondline thickness in a subsequently produced lens wafer. The exact size of the standoffs  8  may vary as required for a specific application. Generally, a desired range of the standoff  8  size is approximately 10-300 μm in height with a round diameter of about 0.5-1.5 mm. 
         [0024]    In the embodiment shown in  FIG. 1 , three standoffs  8  are positioned in a tripod layout configuration. By using only three standoffs  8  in this layout, the standoffs  8  may be incorporated into a master wafer  4  relatively easily and at low cost. It should be understood that the master wafer  4  may be constructed having a different number of standoffs  8  ranging from one to a larger number of standoffs  8 , for example, up to the number of lenses on the master wafer  4  or more. Likewise, although the standoffs  8  are illustrated in a tripod layout, other positioning may be employed; all that is required is that the configuration provides for a balanced support base for stacking additional wafers thereupon when the standoffs are formed on a lens wafer. 
         [0025]    As is known in the art, the master wafer  4  is used to create a negative stamping template, also referred to as a sub-master wafer. The master wafer  4  may be used to create the sub-master wafer through any of various known techniques, including but not limited to depositing a plating film comprising nickel or some other suitable material on the master wafer  4  to form an intermediate negative sub-master wafer. Other known techniques include pressing the master wafer  4  into a moldable material to form a sub-master wafer. Still other known techniques include depositing an ultraviolet curable or thermally curable polymer material onto the master wafer  4  and replicating the master wafer  4  using known replication techniques to form a sub-master wafer. Alternatively, a sub-master wafer may be created directly without the use of a master wafer  4  by diamond turning or other known techniques. 
         [0026]    It should be noted that when a master wafer  4  is used to produce a sub-master wafer which is in turn used to produce a lens wafer, that the produced lens wafer will have the exact same lens shape and standoff configuration as in the master wafer  4 . Thus,  FIG. 1  depicts both master wafer  4  as well as a resulting lens wafer. 
         [0027]      FIG. 2  shows an embodiment of a sub-master wafer  10  having a plurality of standoff dies  20  and lens dies  25  created using and corresponding to the master wafer  4  standoffs  8  and lens shapes  5 . The sub-master wafer  10  is used as a stamp to form lens wafers, as is known in the art. Referring to  FIGS. 3 and 4 , which show a cross sectional view of a stamping operation using a portion of a sub-master wafer  10 , the sub-master wafer  10  is pressed into lens material  40  formed on a surface of a lens wafer  30  to form lenses  65  on the lens wafer  30  which correspond to the lens dies  25  on the sub-master wafer  10 . The standoff dies  20  on the sub master wafer  10  will form standoffs  60  on the lens wafer  30  simultaneously with the lens  65  formation. The completed position of a lens wafer  30  having both lenses  65  and standoffs  60  is shown in  FIG. 4 . 
         [0028]    Standoffs may be formed in various shapes, including but not limited to cylindrical, spherical, rectangular or other shapes. Standoffs may alternatively be formed as an interconnected raised wall. In an embodiment shown in  FIG. 5 , standoff walls  70  are formed on a master wafer  14  encompassing lens shapes  67  in adjoining perimeters. In this embodiment, the lens shapes  67  occupy a space  75  enclosed by a perimeter of a standoff wall  70 . The master wafer  14  shown in  FIG. 5  can then be used to form a sub-master wafer  15  ( FIG. 5B ), which in turn can be used to form lenses and associated wall standoffs in a lens wafer, using, for example, the process shown in  FIGS. 3 and 4 . The standoff wall  70  of master wafer  14  may be designed in various ways, including but not limited to a standoff wall  70  encompassing a group of lenses collectively ( FIG. 5C ), encompassing select lenses individually ( FIG. 5D ), or encompassing each individual lens respectively. 
         [0029]    In addition to having different shapes, standoffs may also be formed in various positional layouts. In an embodiment shown in  FIG. 6 , standoffs  80  are formed on a master wafer  72  as cross-shaped alignment marks positioned to provide marking for aligning a stamped lens wafer with other wafers for wafer stacking purposes. 
         [0030]    The number of standoffs may vary as desired. For example, to further increase the stability and uniformity of the bondline thickness, the number of standoffs can be increased as shown in  FIG. 7 . In this embodiment, the standoffs  50  are included throughout the master wafer  52  arranged in columns alternating with columns of lens shapes  55 . The lens wafer formed using master wafer  52  and a negative sub-master wafer looks identical to master wafer  52  and therefore is also illustrated by  FIG. 7 . 
         [0031]    Use of the standoffs in attaching a spacer wafer to a lens wafer is illustrated in  FIG. 8 , which shows a sectional side view of a lens wafer  30  formed using and corresponding to the master wafer  52  of  FIG. 7 . The standoff  50  height S may be, for example, within the range of 10-300 μm. An adhesive for example, an epoxy  100 , is deposited in areas of the lens wafer outside areas where the lenses  55  are present. The epoxy is provided over the standoffs  50  such that an excess amount X remains above the standoff  50 .  FIG. 9  shows the application of the spacer wafer  120  to the lens wafer  30 . The spacer wafer  120  is pressed into the epoxy  100  towards the lens wafer  30 . Due to the standoffs  50 , the bondline thickness of the epoxy  100  is uniformly equal to the height S ( FIG. 8 ) of the standoff  50 , across the entirety of the spacer wafer and lens wafer. 
         [0032]    Standoffs may also be incorporated on one or both sides of a lens wafer. As shown in  FIG. 10 , a lens wafer  35  may be formed having lenses on both sides and may be attached to other wafer level structures, such as another lens wafer. As shown in  FIG. 10 , as one example, a convex lens  110  can be produced on one side of a lens wafer  35 , a concave lens  115  on the other side, and accompanying standoffs  90  on one side and standoffs  95  on the other side. Lens wafer  35  is suitable for use in multiple level waver level packaging of imager modules. 
         [0033]      FIG. 11  shows an embodiment of a imager module  130  having wafer level optics stack incorporating standoffs  90 , 95  as described above. Imager module  130  comprises two lens wafers  140  and  35 , a spacer wafer  200 , and an imager circuit wafer  180 . In this embodiment, a first lens wafer  140  is on the top of the imager module  130  stack. Lens wafer  140  includes convex lens  150  on one side and concave lens  160  on the other side, and is positioned directly above lens wafer  35  such that lenses  150 , 160  are in vertical alignment with lenses  110 , 115  along a common focal axis. A second lens wafer  35  is positioned between the first lens wafer  140  and the spacer wafer  200  in the imager module  130  stack. The second lens wafer includes convex lens  110  on one side, concave lens  115  on the other side, and standoffs  90 ,  95  on either side, to control the bondline thickness in adhesive layers  165 ,  170  between lens wafer  140 ,  35  bonding areas. Spacer wafer  200  is positioned between the second lens wafer.  135  and the imager circuit wafer  180 . Spacer wafer  200  is attached to imager circuit wafer  180  through adhesive layer  210 . Imager circuit wafer  180  is positioned at the bottom of imager module  130  stack, and includes an imager circuit having a pixel array  190  for capturing a digital image through light received through lenses  110 ,  115 ,  150 , and  160 . 
         [0034]    It should be noted that although  FIG. 11  shows a cross section of an imager module incorporating standoffs at the bonding areas between the lens wafers  140 , 135 , other cross-sections may show attachments at bonding areas that comprise an adhesive layer without standoffs  90 ,  95 , the adhesive layer having a thickness uniform to the height of the standoffs  90 ,  95 . 
         [0035]    Standoffs  90 ,  95  provide a uniform horizontal alignment among lens wafers  140 ,  35  and spacer wafer  200 , thereby helping to place the focal points of the lenses  110 ,  115 ,  150 , and  160  at the desired locations to provide incoming light to the imager circuit  190 . Although the two lens wafers  140 ,  35  are shown, an imager module may be formed having additional lens wafers added to the stack, as well as other layers including color filter arrays, light shields or other layers. Furthermore, standoffs need not be formed on both sides of a lens wafer. Alternatively, standoffs may be formed on one side a lens wafer and, as previously described, may additionally by designed to serve as alignment marks to aid in aligning lens wafer stacks. 
         [0036]    It should be noted that the type of lens shown for the lens wafers, e.g.,  110 ,  115 ,  150 , and  160 , in the various embodiments described are merely exemplary as many different lens designs, shapes and lens materials may be used for a particular imager application. 
         [0037]    While embodiments have been described in detail, it should be readily understood that they are not limited to the disclosed embodiments. Rather the embodiments can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described. Accordingly, the invention is not limited by the described embodiments, but is only limited by the scope of the appended claims.