Abstract:
An electronic device includes a mold package which encapsulates a portion of the electronic device and does not encapsulate another portion of the electronic device to enable a sensing portion of the electronic device to be exposed to a condition to be sensed. In an electronic sensing device having a sensor formed by a substrate such as silicon, a sensor area is not encapsulated, but areas surrounding the sensor area are encapsulated. The area surrounding the sensor area includes one or more trenches or interlock structures formed in the surrounding substrate which receives the mold material to provide an interlock feature. The interlock feature reduces or substantially prevents the mold from delaminating at an interface of the mold and the substrate.

Description:
This application claims the benefit of U.S. Provisional Application No. 61/786,838, filed Mar. 15, 2013, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to an electronic device with a mold package and more particularly to semiconductor device having a robust mold package. 
     BACKGROUND 
     Mold packages are used to encapsulate electronic devices, including semiconductor chips, to protect from undesirable environmental conditions. Such undesirable conditions include light, heat, humidity, dust, and physical shock. In particular, mold packages are widely used in sensor applications and application specific integrated circuit (ASIC) and integrated circuit (IC) packaging since the mold packages provide a low cost package suited for high volume applications. Most packaging includes the use of a black plastic material, typically including epoxy molding compounds. While many electronic components are completely covered by the mold, except for the contacts extending from the component itself, sensors of many types require a mold package that does not completely cover the device. 
     Many sensors directly interact with the environment being sensed and therefore require direct access to the sensed environment. Such sensors include gas-sensors, pressure sensors, bio-sensors, finger-print sensors, and humidity sensors. For these types of sensors, an “exposed-die package” is used which provides a mold which does not cover a predetermined area of the sensor which includes the structure of the sensor providing the sensing function. The drawback of such packages, however, is that since the entire device is not covered by the mold, there is a high probability of mold-delamination at an interface of the mold and the sensor device. Where devices are formed of silicon, the mold-silicon interface (or whatever top-most material is used on the sensor) near the exposed sensor-area can delaminate from the silicon. 
     Consequently, there is a need for a mold package that reduces or substantially prevents the occurrence of delamination between the mold and the sensor. 
     SUMMARY 
     The present disclosure relates to the field of encapsulation of electronic devices, and in particular the partial encapsulation of electronic sensing devices in which a portion of the device is unencapsulated to enable direct sensing of a sensed condition. In particular, the present disclosure relates to a device and method of manufacturing a reliable mold package for sensors, especially for exposed die packaged sensors. 
     In an electronic device with an exposed sensor, the exposed sensor is not encapsulated. In sensing devices made of silicon, a sensing portion is formed in a silicon substrate, as is known by those skilled in the art. The area of the silicon substrate surrounding the sensing portion, however, includes a silicon wafer surface, if silicon is used as a substrate, which provides no sensing function and consequently is covered by the mold. In other embodiments other substrates are used. By introducing a trench-like structure or interlock on the substrate surface adjacent to a sensor surface or area, an interlock structure is provided. The interlock structure increases the adhesion between the mold material and the substrate surface. The increased adhesion reduces the occurrence of a delamination of the mold material from the substrate surface. The potential for delamination is reduced or substantially eliminated. 
     The trench-like structure is configured to provide a predetermined amount of exposed surface area to provide a contact area between the mold material and the substrate adequate for proper adhesion. By increasing the amount of exposed surface area and configuring the shape of the trench, a desired amount of mechanical coupling between the mold and the substrate is provided. The configuration of the trench and the interface between the mold and the trench also provides an interlocking mechanism. When the substrate has been configured to interlock with the mold, the exposed die mold package is prepared for exposure to harsh environments. In addition, the reliability of the sensor package is improved. 
     The described embodiments relate to the field of microelectromechanical systems (MEMS) sensors and specifically a robust and reliable packaging of MEMS sensors and a method of fabricating MEMS sensors having an interlocking mold. 
     A sensor package in one embodiment includes a sensor portion including an upper surface defining a sensor interlock feature, and a mold package including a lower surface defining a mold package interlock feature, wherein the mold package interlock feature is interlocked with the sensor interlock feature. 
     A method of forming a sensor package in one embodiment includes forming a sensor portion including an upper surface, defining a sensor interlock feature in the upper surface, forming a mold package including a lower surface defining a mold package interlock feature, and interlocking the mold package interlock feature with the sensor interlock feature. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a prior art sensor package including a mold and an exposed sensor formed in a silicon substrate. 
         FIG. 2  is schematic cross-sectional view of a sensor package including a mold and an exposed sensor formed in a silicon substrate having an interlock structure. 
         FIG. 3  is a schematic cross-sectional view of a capacitive pressure sensor including an interlock structure. 
         FIGS. 4   a  and  4   b  are schematic cross-sectional views of different configurations of an interlock structure formed in a substrate. 
         FIG. 5  is a schematic cross-sectional view of the pressure sensor of  FIG. 3  including a die mold encapsulation. 
         FIGS. 6   a  and  6   b  are schematic cross-sectional views of the configurations of  FIGS. 4   a  and  4   b  including a die mold encapsulation. 
     
    
    
     DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that the present disclosure includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the disclosure as would normally occur to one of ordinary skill in the art to which this disclosure pertains. 
       FIG. 1  illustrates a schematic cross-sectional view of a prior art sensor package  10  including an exposed die mold package  12  and an exposed sensor  14  formed in a silicon substrate  16 . The mold package  12  interfaces with a surface of the substrate  16  at a first location  18  and a second location  20 . Each of these locations  18  and  20  are susceptible to delamination of the mold package  12  from the substrate  16 . While two locations  18  and  20  are illustrated, other areas of delamination between the substrate  16  and the mold package  12  are possible. Mold delamination is possible at any location where the mold package  12  interfaces with the substrate  16 . 
       FIG. 2  illustrates a schematic cross-sectional view of a sensor package  26  including a mold package  28  and an exposed sensor  30 . Similar to  FIG. 1 , the sensor  30  is formed in a substrate  32  upon which the mold package  28  is disposed. In the embodiment of  FIG. 2 , however, the sensor package includes an interlocking feature  34  and an interlocking feature  36 . Each of the interlocking features include an interlocking structure  38  provided by the mold package  28  and an interlocking structure  40  provided by the substrate  32 . The interlocking feature  40  provided in the substrate  32  is a recess formed in the surface of a substrate. The interlock structure  38  provided by the mold package is formed by the flow of mold material, which forms the mold package, into the interlock structure  40  or the recess of the substrate  32 . 
       FIG. 3  illustrates schematic cross-sectional view of a capacitive pressure sensor  42  illustrating an interlock structure  44  formed in the surface of the substrate. As illustrated in  FIG. 3 , the silicon is etched to form a trench  46  on the surface of a portion of a substrate  48 . The trench  46 , in different combinations, provides interlocking structures which include apertures, troughs, channels or other formations in the surface of the silicon having sidewalls and a bottom wall of silicon. Once the trench  46  having silicon walls is formed, an oxide layer  49  is deposited on the upper surface of the substrate  48  which also fills the trench. The illustration of  FIG. 3  does not show the portion of the oxide layer which is removed from the horizontally depicted upper surface of the substrate  48 . The oxide deposition fills in the trench  46  to form an oxide column which is then etched using a photoresist layer (not shown). Etching of the oxide column provides the completed interlock structure  44 . 
       FIGS. 4   a  and  4   b  are schematic cross-sectional views of different configurations of interlock structures formed in a portion of a substrate.  FIG. 4   a  illustrates the interlock structure  44  of  FIG. 3  and another interlock structure  50 . The interlock structure  50  is formed similarly as the structure of  FIG. 4   a , except that a bottom wall  52  is formed of an oxide layer  52  formed prior to the addition of a silicon layer  54  through which an etched trench is formed. The etched trench includes sidewalls having a deposited oxide layer  56  formed as described above with respect to interlock structure  44 . As further illustrated in  FIG. 4   b , a plurality of interlock structures  60 ,  62 , and  64  are formed in layer of silicon  66  disposed on an oxide layer  68 . The interlock structure  60 ,  62 , and  64  each include sidewalls of bare silicon and are not covered with an oxide layer. A floor for each of the interlock structures, however, is provided by the oxide layer  68 . In different embodiments, the sidewalls can be include parallel sidewalls (structure  60 ) inwardly oriented sidewalls (structure  62 ), and outwardly oriented sidewalls (structure  64 ). 
       FIG. 5  is a schematic cross-sectional view of the pressure sensor  42  of  FIG. 3  including a die mold encapsulation  70 . As illustrated, the interlock structure  44  is filled with the mold compound and an interlock feature is formed between the mold encapsulation and the trench  46  of pressure sensor  42 . The trench  46  having the sidewalls and bottom wall of oxide is completely filled in the illustrated embodiments. In other embodiments, the interlock structure  44  is not completely filled with mold compound, but includes a sufficient amount of compound to hold the encapsulation to the substrate. 
       FIGS. 6   a  and  6   b  are schematic cross-sectional views of the configurations of  FIGS. 4   a  and  4   b  including a die mold encapsulation.  FIGS. 6   a  and  6   b  show the same structures as  FIGS. 4   a  and  4   b  after the molding process has been completed. The mold compound fills the structures and provides an interlock feature between the mold and the sensor to interlock the mold to the device. The interlock structures do not need to be filled completely as shown. As can be seen in  FIG. 6   a , a one or more portions  74  of the oxide layer  56  on the vertical walls of the troughs overhangs a lower portion  76  of the mold material. In this way, the portions  74  provide a structure which increases the ability of the interlock structure  50  to hold the mold in place. Likewise in  FIG. 6   b , the sidewalls of the structure  62  are closer to one another at a top portion  78  than at a bottom portion  80 . The structure  62  therefore includes an overhanging structure which increases the ability of the interlock structure  62  to hold the mold in place which reduces or eliminates the occurrence of delamination. 
     In other embodiments, the trench interlock structures can be realized as a closed ring structure surrounding the sensor area, or as a structure of single trenches and or trench arrays. In other embodiments the interlock structures include one or more circular or oval depressions formed in the substrate which are spaced from one another. In still other embodiments, the interlock structures include apertures configured to receive mold material to provide an interlock feature. 
     The sensors and devices which include the interlock structure for the mold package of the present disclosure can be embodied in a number of different types and configurations. The following embodiments are provided as examples and are not intended to be limiting. 
     A sensor in one embodiment includes a trench structure for realizing a mold-interlock. In another embodiment, a mold interlock for exposed-die mold package is provided for sensors, ASICs, and MEMS devices. In yet another embodiment, an interlock structure is provided for use in gas-sensors, pressure sensors, bio-sensors, finger-print sensors, humidity sensors. 
     A sensor in one embodiment includes a trench structure with increasing diameter and/or critical dimension (trench width if a straight line for example) from a top of a substrate toward an interior or bottom surface of a substrate. In another embodiment, a sensor includes a trench structure with multiple different critical dimensions from a top surface toward a bottom surface of a substrate such as wider and narrower. A sensor in yet another embodiment includes a trench structure being completely filled with mold-compound. In yet another embodiment, a sensor includes a trench structure being at least partly filled with mold-compound. A sensor in yet another embodiment includes a trench structure formed as a closed perimeter surrounding the sensor area. 
     In one embodiment, a sensor includes a plurality of closed perimeters disposed around the sensor area, wherein adjacent trenches define perimeters of different sizes. A sensor in another embodiment includes a trench structure realized as an array of single/insulated trenches. A sensor in yet another embodiment includes a combination of closed perimeters and insulated trenches. In yet another embodiment, a sensor includes a closed or open perimeter defining a meandering path to increase the amount of interlocking surface area. 
     While the disclosure has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the disclosure are desired to be protected.