Abstract:
A sensor package that includes a substrate with opposing first and second surfaces. A plurality of photo detectors are formed on or under the first surface and configured to generate one or more signals in response to light incident on the first surface. A plurality of contact pads are formed at the first surface and are electrically coupled to the plurality of photo detectors. A plurality of holes are each formed into the second surface and extending through the substrate to one of the contact pads. Conductive leads each extend from one of the contact pads, through one of the plurality of holes, and along the second surface. The conductive leads are insulated from the substrate. One or more trenches are formed into a periphery portion of the substrate each extending from the second surface to the first surface. Insulation material covers sidewalls of the one or more trenches.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 15/017,506, filed Feb. 5, 2016, which claims the benefit of U.S. Provisional Application No. 62/120,255 filed Feb. 24, 2015. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to image sensors, and more particularly to an image sensor that is packaged in a manner that reduces induced stress. 
     BACKGROUND OF THE INVENTION 
     Silicon wafers are hard, brittle and stable. However, a silicon wafer is only stable before it is processed to form integrated circuits thereon (e.g. doping, processing, thinning, having layers of material/structure added to it, etc.). After that, the wafer will become unstable, can warp severely especially when the wafer is very thin and has unbalanced structural support, making the wafer extremely frail and susceptible to mechanical stress damage. 
     As the wafer diameter gets larger to enhance productivity/efficiency and chips get thinner to meet the requirements for heat dissipation, die stacking, reduced electrical resistance and low profile devices, such thin chips on large wafers will suffer ever-greater magnitude of stresses than ever before. These mechanical stress issues are especially severe for image sensor wafers (i.e. wafer on which image sensors are formed). The active side of an image sensor wafer has layers of material and structures formed thereon, which can include passivation, low-k dielectric layers, microlenses, color filters, conductive circuits, optical enhancements, light shielding, etc. These layers and structures not only make the silicon wafer unstable, they themselves are even more susceptible to the same mechanical stress and can become damaged. 
     Additionally, the active side of an image sensor wafer can be encapsulated with a protective substrate, which includes stand offs (dam) structures to space it from the wafer. The stand offs are bonded to the surface layer and introduce mechanical stress to the surface layer, together with the buildup of enormous amounts of mechanical stress during wafer thinning and dicing steps, which can cause cracking, delamination and many other defects on the surface layers and/or silicon substrate. 
     It is known in the art to make a pre-cut (partial dicing) to avert/release mechanical stress build up. Processing such as Dice Before Grinding (DBG) includes making a partial cut into the silicon wafer, thinning the other side of the wafer, using plasma etch to relieve stress build up in the wafer, and then making the final singulation cut. However, a limitation of DBG processing or similar processing is that such processing is for non-packaged semiconductor silicon wafers. What is needed is a method and structure for mechanical stresses relief that is compatible with and is part of the Wafer Level Packaging (WLP) process (i.e. packaging of the integrated circuits before wafer singulation). 
     BRIEF SUMMARY OF THE INVENTION 
     The aforementioned problems and needs are addressed by a method of forming a sensor package includes providing a sensor chip that includes a first substrate with opposing first and second surfaces, a plurality of photo detectors formed on or under the first surface of the first substrate and configured to generate one or more signals in response to light incident on the first surface of the first substrate, and a plurality of contact pads formed at the first surface of the first substrate and which are electrically coupled to the plurality of photo detectors. A plurality of holes are formed into the second surface of the first substrate, wherein each of the plurality of holes extends through the first substrate and to one of the contact pads. A plurality of conductive leads are formed each extending from one of the contact pads, through one of the plurality of holes, and along the second surface of the first substrate. One or more trenches are formed into a periphery portion of the first substrate each extending from the second surface to the first surface. Insulation material is formed that covers sidewalls of the one or more trenches. 
     A method of forming a plurality of sensor packages includes providing a sensor chip that includes a first substrate with opposing first and second surfaces, and a plurality of sensors formed thereon, wherein each sensor includes a plurality of photo detectors formed on or under the first surface of the first substrate and configured to generate one or more signals in response to light incident on the first surface of the first substrate, and a plurality of contact pads formed at the first surface of the first substrate and which are electrically coupled to the plurality of photo detectors. A plurality of holes are formed into the second surface of the first substrate, wherein each of the plurality of holes extends through the first substrate and to one of the contact pads. A plurality of conductive leads are formed each extending from one of the contact pads, through one of the plurality of holes, and along the second surface of the first substrate. A dam structure is formed on the first surface of the first substrate and around but not over the plurality of photo detectors. A second substrate is formed on the dam structure, wherein the second substrate extends over the plurality of photo detectors, and wherein the dam structure and the second substrate form a sealed cavity over the plurality of photo detectors for each of the sensors. One or more trenches are formed into the first substrate at a periphery portion of each of the sensors extending from the second surface, to the first surface, and into the dam structure. Insulation material is formed that covers sidewalls of the one or more trenches. The first substrate is singulated into separate die at the trenches, wherein each die includes one of the sensors. 
     Other objects and features of the present invention will become apparent by a review of the specification, claims and appended figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-5  are side cross sectional views illustrating the steps in forming the image sensor package of the present invention. 
         FIGS. 6A-6F  are side cross sectional view illustrating alternate configurations of the trenches formed into the silicon substrate. 
         FIGS. 7A-7B  are side cross sectional view illustrating alternate configurations of the via holes into the silicon substrate. 
         FIGS. 8-14  are side cross sectional views illustrating the steps in forming the image sensor package of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a sensor package with steeping features on the sides of the package. The steeping feature is the result of making a pre-cut from the backside of the sensor wafer (e.g., image senor, light sensor, biometric sensor, etc.) instead of from the front side (active side). The steeping feature is encapsulated by a layer of encapsulant so that no silicon and/or passivation layers are exposed to the external elements. The bond pads of the image sensor are rerouted to the backside of the image sensor where interconnect bumps are formed. The front side of the image sensor is encapsulated by a permanent protective substrate using a dam structure that forms a cavity over the sensor active area. 
       FIGS. 1-14  illustrate the process of forming the image sensor assembly. The process begins by providing an image sensor wafer  1  having a semiconductor substrate  10 , a plurality of bond pads  12  and active areas with photo detectors (i.e. photodiodes)  14  and circuit layers  16  that support the operation of the photodiodes  14 , as illustrated in  FIG. 1 . The photodiodes  14  generate electrical signals in response to light incident on the sensor active area. Those signals are eventually coupled to the bond pads  12  for off chip signaling. 
     The image sensor  1  preferably includes a surface layer  18  that can include passivation, a low-k dielectric layer, microlenses and color filters  20 , conductive circuits, optical enhancements, light shielding, etc. The image sensor wafer containing many image sensors  22  (each with its own photodiodes, circuit layers, bond pads, and surface layer) as shown in  FIG. 2  is well known in the art, and not further described herein. 
     The sensor active area is encapsulated by a permanent protective substrate  24  mounted to the substrate by a dam structure  26 . The protective substrate  24  is preferably optically transparent. The dam  26  is preferably formed on the optically transparent material by deposition of polymer material and selective removal of the polymer material. Adhesive is applied to the dam  26 , which is then bonded to the image sensor wafer. The dam  26  and substrate  24  form a sealed cavity  28  over the active area of the image sensor  22 , as shown in  FIG. 3 . The silicon on the back of the image sensor wafer substrate  10  can be thinned by mechanical grinding, chemical mechanical polishing (CMP), wet etching, atmospheric downstream plasma (ADP), dry chemical etching (DEC) or any other appropriate silicon thinning methods, as shown in  FIG. 4 . After the thinning process, an optional plasma-etching step can be made to release stress buildup in the wafer (however this will not release all the stress that has built up on the surface layer  18 ). 
     Portions of the silicon on the backside of the image sensor wafer substrate  10  are selectively removed at a scribe line  30  separating the image sensors  22  (forming trenches  32  that extend at least partially through substrate  10  and via holes  34  that extend through the substrate  10  to expose bond pads  12 ), as shown in  FIG. 5 . The silicon is selectively removed using lithography and plasma etching methods or any other silicon etching methods that are well known in the art. The image sensor bond pads  12  should be exposed from the backside of the image sensor wafer by the via holes  34 , each of which extends all the way from the wafer back surface to one of the bond pads  12 . The via holes  34  can be tapered or not tapered. The trenches  32  can be tapered or not, and can have optional secondary trench portions  32   a  that are tapered or not tapered, and can extend partially or completely through the wafer substrate  10 , as shown in various configurations in  FIGS. 6A-6F . Specifically,  FIGS. 6A and 6B  illustrate tapered and non-tapered trenches  32 , respectively, etched partially through the silicon wafer which do not extend all the way to and expose the surface layer  18 .  FIGS. 6C-6F  illustrate different variations of trench taper, each of which includes a trench  32  partially through the wafer, and a secondary trench portion  32   a  of trench  32  that extends all the way to and exposing the surface layer  18 . In all the configurations of  FIGS. 6C-6F , the trench  32  has a step (i.e., shoulder) in its silicon sidewall where trench portion  32   a  begins.  FIGS. 7A and 7B  show tapered and non-tapered via hole configurations, respectively. 
     A mechanical dicer or laser is used to extend the trenches  32 / 32   a  through the surface layer(s)  18  and partially into the dam  26  (i.e. extending the trenches entirely through the silicon wafer and surface layer(s) and partially into the dam along the scribe line  30 ), as shown in  FIG. 8 . This will relieve the physical stress on the surface layer(s) and prevent it from cracking during the die singulation step later in the processing.  FIG. 9  shows the same configuration except that the secondary trench portion  32 a through the substrate  10  is tapered. 
     A layer of silicon dioxide, silicon nitride or any other appropriate passivation/isolation layer  36  can be conformably deposited over the backside of the silicon wafer using methods such as physical vapor deposition (PVD) or by spin/spray coating system. The passivation/isolation layer  36  is formed or selectively etched so that it lines trenches  32  and holes  34  except that the bond pads  18  are left exposed at the ends of the via holes  34 , as shown in  FIG. 10 . 
     Conductive material is deposited over the passivation layer  36  using physical vapor deposition and plating or any other appropriate conductive layer deposition methods. The conductive layer can be a stack of titanium, copper, nickel and gold or any other appropriate conductive material. The conductive layer is selectively removed using photolithography and etching processes, leaving conductive leads  38  of the conductive material that each extend from one of the bond pads  12 , along the via hole sidewall, and along the backside surface of the substrate  10 , so as to electrically reroute the bond pad  12  to the backside of the image sensor through the via hole  34 , as shown in  FIG. 11 . 
     Encapsulant  40  is deposited over the backside of the substrate  10  covering the wafer backside and filling trenches  32  and holes  34 . The encapsulant can be a polymer or other dielectric material. The encapsulant is selectively removed using a photolithography process to expose selective portions  38   a  of the conductive leads  38  (referred to as rerouted contact pads), as shown in  FIG. 12 . While the encapsulant is shown as completely filling all the backside trenches/holes, the encapsulant could instead be a thin conformal layer over the backside structures which does not completely fill the trenches/holes. The encapsulant can be deposited by spray coating. 
     Electrical interconnects  42  are formed on the rerouted contact pads  38   a . Electrical interconnects  42  can be ball grid array (BGA), plated bump, conductive adhesive bump, gold stud bump or any other appropriate interconnection methods. Preferably, the interconnect bumps are solder ball grid array. Wafer level dicing/singulation of components through the scribe lines that run through trenches  32  is then done using mechanical blade dicing equipment or any other appropriate processes, which extends through the encapsulation  40 , part of the dam  26  and the transparent substrate  24 . This singulation involves no cutting through the silicon substrate, and only partially through the dam  26 , as shown in  FIG. 13 . 
     The final singulated die sensor package is shown in  FIG. 14 . The sides of the sensor die are encapsulated so that there is no exposed silicon of substrate  10  (i.e. the side portions of substrate  10  are protected/sealed by insulation layer  36  and encapsulation  40 ). Further, the sensor active area is never exposed once the dam  26  and transparent substrate  24  are formed thereon early in the process. 
     It is to be understood that the present invention is not necessary limited to the embodiment(s) described above and illustrated herein, but encompasses any and all variations falling within the scope of the claims. For example, the dam structure can be omitted, whereby the cavity is formed into the bottom surface of the protective substrate by etching of the substrate material. References to the present invention herein are not intended to limit the scope of any claim or claim term, but instead merely make reference to one or more features that may be covered by one or more claims. Materials, processes and numerical examples described above are exemplary only, and should not be deemed to limit any eventual claims. Further, not all method steps need be performed in the exact order illustrated, but rather in any order that allows the proper formation of the packaged image sensor. Lastly, single layers of material could be formed as multiple layers of such or similar materials, and vice versa. 
     It should be noted that, as used herein, the terms “over” and “on” both inclusively include “directly on” (no intermediate materials, elements or space disposed therebetween) and “indirectly on” (intermediate materials, elements or space disposed therebetween). Likewise, the term “adjacent” includes “directly adjacent” (no intermediate materials, elements or space disposed therebetween) and “indirectly adjacent” (intermediate materials, elements or space disposed there between), “mounted to” includes “directly mounted to” (no intermediate materials, elements or space disposed there between) and “indirectly mounted to” (intermediate materials, elements or spaced disposed there between), and “electrically coupled” includes “directly electrically coupled to” (no intermediate materials or elements there between that electrically connect the elements together) and “indirectly electrically coupled to” (intermediate materials or elements there between that electrically connect the elements together). For example, forming an element “over a substrate” can include forming the element directly on the substrate with no intermediate materials/elements therebetween, as well as forming the element indirectly on the substrate with one or more intermediate materials/elements therebetween.