Patent Publication Number: US-2015069537-A1

Title: Package-on-package semiconductor sensor device

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to semiconductor packaging and, more particularly to a package-on-package type semiconductor pressure sensor. 
     Semiconductor sensor devices, such as pressure sensors, are well known. Such devices use semiconductor pressure-sensing dies. These dies are susceptible to mechanical damage during packaging and environmental damage when in use, and thus they must be carefully packaged. Further, pressure-sensing dies, such as piezo resistive transducers (PRTs) and parameterized layout cells (P-cells), do not allow full encapsulation because that would impede their functionality. 
       FIG. 1(A)  shows a cross-sectional side view of a conventional semiconductor sensor device  100  having a metal lid  104  and a pressure sensor die  106 .  FIG. 1(B)  shows a perspective top view of the sensor device  100  without the lid  104  and without a gel  114  coating over the pressure sensor die, and  FIG. 1(C)  shows a perspective top view of the lid  104 . 
     As shown in  FIG. 1 , the pressure sensor die (P-cell)  106 , an acceleration-sensing die (G-cell)  108 , and a micro-controller unit die (MCU)  110  are mounted to a flag  112  of a lead frame, electrically connected to package leads  118  by bond wires (not shown), and covered by the pressure-sensitive gel  114 , which enables the pressure of the ambient atmosphere to reach the pressure-sensitive active region on the top side of the P-cell  106 , while protecting all of the dies  106 ,  108 ,  110  and the bond wires from mechanical damage during packaging and environmental damage (e.g., contamination and/or corrosion) when in use. The entire die/substrate assembly is encased in a molding compound  102  and covered by the lid  104 . The lid  104  has a vent hole  116  that exposes the gel-covered P-cell to ambient atmospheric pressure outside the sensor device. 
     One problem with the design of sensor device  100  is the high manufacturing cost due to the use of a pre-molded lead frame, the metal lid  104 , and the large volume of pressure-sensitive gel  114 . Accordingly, it would be advantageous to have a more-economical way to assemble a pressure sensor device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure are illustrated by way of example and are not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the thicknesses of layers and regions may be exaggerated for clarity. 
         FIG. 1  shows a conventional packaged semiconductor sensor device having a metal lid; 
         FIG. 2  shows a semiconductor sensor device in accordance with an embodiment of the disclosure; and 
         FIGS. 3(A)-3(J)  illustrate one possible process for manufacturing the sensor device of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Detailed illustrative embodiments of the present disclosure are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present disclosure. Embodiments of the present disclosure may be embodied in many alternative forms and should not be construed as limited to only the embodiments set forth herein. Further, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the disclosure. 
     As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It further will be understood that the terms “comprises,” “comprising,” “has,” “having,” “includes,” and/or “including” specify the presence of stated features, steps, or components, but do not preclude the presence or addition of one or more other features, steps, or components. It also should be noted that, in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
     In one embodiment, the present invention provides a method of assembling a semiconductor sensor device, and in another embodiment is the resulting semiconductor sensor device. A micro controller unit (MCU) die is mounted on a substrate or lead frame. An interposer is mounted on the MCU die. The MCU die and a first portion of the interposer are encapsulated in a molding compound, leaving a second portion of the interposer exposed. A pre-packaged pressure sensor then is mounted onto the exposed, second portion of the interposer. The interposer includes through metal vias or other wiring patterns that allow the interposer to provide electrical interconnection between the MCU die and the pre-packaged pressure sensor. 
       FIG. 2  shows a cross-sectional side view of a packaged semiconductor sensor device  200  in accordance with an embodiment of the present invention. The exemplary configuration of the sensor device  200  forms a no-leads type package such as a quad flat no-leads (QFN) package. Note that alternative embodiments are not limited to QFN packages, but can be implemented for other package types, such as (without limitation) ball grid array (BGA) packages, quad flat packages (QFP) or other leaded packages. 
     The sensor device  200  comprises a lead frame  202  having a die paddle  204  and multiple metal leads  206  separated by and embedded within an electrically insulating molding compound  208 . The lead frame  202  may be formed of copper, an alloy of copper, a copper plated iron/nickel alloy, plated aluminum, or the like. Often, copper leads are pre-plated first with a nickel base layer, then a palladium mid-layer, and finally with a very thin, gold upper layer. The molding compound  208  may be an epoxy or other suitable material. The lead frame  202  and molding compound  208  together comprise a pre-molded lead frame that may be formed and obtained from a supplier as opposed to being formed at the sensor device assembly site. 
     The lead frame  202  functions as a substrate to which other elements of the sensor device  200  are mounted. More specifically, an MCU die  210  and an acceleration-sensing die (a.k.a. G-cell)  212  are mounted on and attached to the die paddle  204 . Wire-bond pads on the MCU  210  are electrically connected to one or more of the leads  206  with bond wires  214 , and one or more other wire-bond pads on the MCU  210  are electrically connected to one or more wire-bond pads on the G-cell  212  with bond wires  216 . The G-cell  212 , which is an optional component, is designed to sense gravity or acceleration in one, two, or three axes, depending on the particular implementation. The bond wires  214  and  216  are formed from a conductive material such as aluminium, silver, gold, or copper, and may be either coated or uncoated. Note that, in alternative designs, the MCU  210  and/or G-cell  212  can be electrically connected to the leads  206  using suitable flip-chip, solder-bump techniques instead of or in addition to wire bonding. 
     Conventional, electrically insulating die-attach adhesive (not shown) may be used to attach the MCU  210  and G-cell  212  to the die paddle  204 . Those skilled in the art will understand that suitable alternative means, such as die-attach tape, may be used to attach some or all of these dies. 
     An interposer  220  is mounted on a top surface of the MCU  210  with bump interconnections  218 , and a pre-packaged pressure sensor  224  is mounted on a top surface of the interposer  220  with other bump interconnections  222 . In one implementation, the interposer  220  comprises a single metal layer sandwiched between two insulating layers with one or more metal vias through the insulating layers. The metal vias and patterned metal features in the metal layer along with corresponding bump interconnections  218  and  222  provide electrical interconnections between the MCU  210  and the pre-packaged pressure sensor  224 . In another embodiment, the interposer  220  may comprise a substrate formed of a non-conductive material (e.g., ceramic) with through metal vias. 
     The pre-packaged pressure sensor  224 , which may itself be a BGA package, comprises a pressure-sensing die (i.e., P-cell)  226  mounted within a package housing  228 . The P-cell  226  is designed to sense ambient atmospheric pressure. The pre-packaged pressure sensor  224  may take various forms, such as the P-cell  226  being electrically connected to leads (not explicitly shown in  FIG. 2 ) in the package housing  228  with bond wires  230 . A pressure-sensitive gel  232  covers the P-cell  226  and bond wires  230  and fills the cavity of package housing  228 . Note that, in alternative implementations, less gel material  232  may be applied within the package housing  228  as long as the pressure-sensitive active region (typically on the top side) of the P-cell  226  and its associated bond wires  230  are covered by the gel  232 . The Pressure-sensitive gel  232  enables the pressure of the ambient atmosphere to reach the active region of P-cell  226 , while protecting P-cell  226  and its associated bond wires  230  from mechanical damage during packaging and environmental damage (e.g., contamination and/or corrosion) when in use. Examples of suitable pressure-sensitive gel  232  are available from Dow Corning Corporation of Midland, Mich. The gel  232 =may be dispensed with a nozzle of a conventional dispensing machine, as is known in the art. A lid  234  having an opening or vent hole  236  is mounted on top of the package housing  228  over the gel-covered P-cell  226 , thereby providing a protective cover for the P-cell. The vent hole  236  allows the ambient atmospheric pressure immediately outside the pre-packaged pressure sensor  224  and therefore immediately outside the sensor device  200  to reach the pressure-sensitive gel  232  and therethrough the active region of the P-cell  226 . Although shown centered in  FIG. 2 , the vent hole  236  can be located anywhere within the area of the lid  234 . The vent hole  236  may be pre-formed in the lid by any suitable fabrication process such as drilling or punching. The lid  234  is formed of a durable and stiff material, such as stainless steel, plated metal, or polymer, so that the P-cell  226  is protected. The lid  234  is sized and shaped depending on the size and shape package housing  228 , which is itself sized and shaped depending on the size and shape of the P-cell  226 . Accordingly, depending on the implementation, the lid  234  may have any suitable shape, such as round, square, or rectangular. 
     The lead frame  202 , MCU  210 , G-cell  212 , bond wires  214  and  216 , and all but a portion of the top surface of the interposer  220  are encapsulated in a molding compound  238 . The molding compound  238  may be a plastic, an epoxy, a silica-filled resin, a ceramic, a halide-free material, the like, or combinations thereof, is known in the art. As explained below in the context of  FIG. 3(G) , depending on the particular implementation, the molding compound  208  of the pre-molded lead frame  202  and the encapsulating molding compound  238  may be applied in a single manufacturing step or in two different manufacturing steps. That is, if applied in a single step then a regular lead frame instead of a pre-molded lead frame is used in the assembly process. 
     Thus, the pre-packaged pressure sensor  224  is electrically connected to the MCU by way of the bumps  222 , interposer  220  and bumps  218 . The MCU  210  functions as a controller for both the G-cell  212  and the P-cell  226  by, for example, controlling the operations of and processing signals generated by these two sensor dies. Note that, in some embodiments, the MCU  210  may implement both the functionality of an MCU and that of one or more other sensors, such as an acceleration-sensing G-cell, in which latter case, the G-cell  212  may be omitted. The MCU  210 , G-cell  212 , and P-cell  226  are well-known components of semiconductor sensor devices and thus detailed descriptions thereof are not necessary for a complete understanding of the invention. 
     The sensor device  200  can be manufactured with less cost than comparable sensor devices, like the conventional sensor device  100  of  FIG. 1  because the sensor device  200  has a smaller lid and uses less pressure-sensitive gel. Furthermore, because the P-cell  226  is pre-packaged within a stand-alone pressure sensor device  224 , the pressure sensor device  224  can be tested independently, prior to being packaged within the sensor device  200 . The interposer prevents direct mold clamping onto the MCU  210 , which reduces the risk of cracks forming in the MCU  210 . 
       FIGS. 3(A)-3(J)  illustrate one possible process for manufacturing multiple instances of sensor device  200  of  FIG. 2 . In particular,  FIG. 3(A)  shows a cross-sectional side view of die paddles  204  and metal lead structures  306  that will eventually form leads  206  of multiple instances of lead frame  202  of  FIG. 2 . Note that, later in the manufacturing process, singulation will sever each lead structure  306  into two leads  206 , one lead for each of two adjacent instances of lead frame  202 . Die paddles  204  and lead structures  306  are all mounted on suitable lead frame tape  302 . 
       FIG. 3(B)  shows a cross-sectional side view of conventional pick-and-place machinery  304  placing MCU and G-cell dies  210  and  212  onto the die paddles  204  of  FIG. 3(A) . 
       FIG. 3(C)  shows a cross-sectional side view of the MCU and G-cell dies  210  and  212  of  FIG. 3(B)  being oven-cured onto die paddles  204 . Note that, depending on the implementation, the attachment or die-bonding of all of the MCU and G-cell dies can be achieved in a single die-bonding process step that includes the curing of the epoxy or other substance (e.g., die-attach tape) used to mount all of those dies in a single pass through a curing cycle (e.g., comprising heating and/or UV irradiation). 
       FIG. 3(D)  shows a cross-sectional side view of the MCU dies  210  of  FIG. 3(C)  after being wire-bonded to both the G-cell dies  212  and to lead structures  306 . Note that (i) the MCU dies can be electrically connected to the lead structures and (ii) the G-cell dies can be electrically connected to the MCU dies all in a single pass though a wire-bonding cycle (or in a single wire-bonding process step). 
       FIG. 3(E)  shows a cross-sectional side view of the pick-and-place machinery  304  placing instances of interposer  220  onto corresponding MCU dies  210  of  FIG. 3(D) . Note that the MCU bond pads (not explicitly shown) are either plated or non-plated. For non-plated wafers, stud bumping can be performed prior to the placement of the interposers  220 . 
       FIG. 3(F)  shows a cross-sectional side view of the interposers  220  of  FIG. 3(E)  being subjected to reflow or oven-curing for thermo compression bonding, depending on the media of interconnection between the interposers and the MCU dies. 
       FIG. 3(G)  shows a cross-sectional side view of the result of film-assisted encapsulation with pin molding, being applied to the sub-assemblies of  FIG. 3(F) . Although not explicitly depicted in the figures, film is pressed onto interposers  220 , mold pins are pressed on the film, and then molding compound is applied to encapsulate and embed all of the existing elements within molding compound  208 / 238 , while leaving a large area on the top of each interposer  220  exposed. The mold pins and the film prevent the molding compound from seeping onto the exposed areas of the interposers. 
     One way of applying the molding compound is using a mold insert of a conventional injection-molding machine, as is known in the art. The molding material is typically applied as a liquid polymer, which is then heated to form a solid by curing in a UV or ambient atmosphere. The molding material can also be a solid that is heated to form a liquid for application and then cooled to form a solid mold. Subsequently, an oven is used to cure the molding material to complete the cross linking of the polymer. In alternative embodiments, other encapsulating processes may be used. 
     Note that, in this implementation, the lead frame molding compound  208  and the encapsulating molding compound  238  result from a single application of molding compound. In an alternative implementation, the lead frame is pre-molded prior to the step of  FIG. 3(A) , in which case the corresponding step of  FIG. 3(G)  would involve the application of only molding compound  238 . In either case, after encapsulation, the mold pins and the film are removed to produce the structure shown in  FIG. 3(G) . 
       FIG. 3(H)  shows a cross-sectional side view of the pick-and-place machinery  304  placing instances of pre-packaged pressure sensor  224  onto corresponding interposers  220  of  FIG. 3(G) . In a preferred embodiment of the invention, the molding compound  238  extends well above a top surface of the interposer  220  such that a cavity in the molding compound  238  is formed over the interposer  220 . The pre-packaged pressure sensor  224  is placed in the cavity. In another preferred embodiment, when seated within the cavity, a top of the pre-packaged pressure sensor  224  is flush with a top surface of the molding compound  238 . 
       FIG. 3(I)  shows a cross-sectional side view of the pre-packaged pressure sensors  224  of  FIG. 3(H)  being subjected to reflow or oven-curing for thermo compression bonding, depending on the media of interconnection between the pre-packaged pressure sensors and the interposers. 
       FIG. 3(J)  shows a cross-sectional side view of the structure of  FIG. 3(I)  after (i) being flipped over and placed onto UV tape  308 , (ii) removal of lead frame tape  302 , and (iii) performance of saw or laser singulation, during which each lead structure  306  is severed into two leads  206  of adjacent instances of sensor device  200  The resulting structure of  FIG. 3(J)  comprises multiple instances of semiconductor sensor device  200  of  FIG. 2  mounted onto UV tape  308 , which can then be safely removed without pulling off any of the pre-packaged pressure sensors  224 . Another method of saw singulation is using a jig to hold the structure, in which case the UV tape is not required. 
     Although not explicitly depicted in the drawings, in real-world manufacturing, a two-dimensional array of different instances of sensor device  200  would be assembled on a multi-device lead frame that consists of a two-dimensional array of different instances of the lead frame structures of  FIG. 3(A) . After assembly, e.g., using the process depicted in  FIGS. 3(A)-3(J) , the multiple sensor devices would then be separated, e.g., in a singulation process involving a saw or laser, to form individual instances of sensor device  200 . 
     As used herein, the term “mounted to” as in “a first die mounted to a die paddle” covers situations in which the first die is mounted directly to the lead frame with no other intervening dies or other structures (as in the mounting of MCU  210  to die paddle  204  in  FIG. 2 ) as well as situations in which the first die is directly mounted to another die, which is itself mounted directly to the die paddle. An example of this latter situation would be an embodiment in which a G-cell die is mounted to an MCU die, which is in turn mounted to a die paddle, in which case, the G-cell die could be said to be “mounted to” the die paddle, albeit via the MCU die. Note that “mounted to” also covers situations in which there are two or more intervening dies between the first die and lead frame. Depending on the situation, the term “mounted” can imply electrical connection in addition to physical attachment, where the electrical connection may be provided by one or more bond wires, one or more solder bumps, and/or any other suitable technique. 
     Although  FIG. 2  shows sensor devices  200  having a P-cell and a G-cell, those skilled in the art will understand that, in alternative embodiments, the G-cell and its corresponding bond wires may be omitted. 
     Although  FIG. 2  shows an embodiment in which a G-cell and an MCU are mounted to a die paddle with the electrical interconnection provided by wire-bonding, those skilled in the art will understand that the electrical interconnection between such dies and paddles can, alternatively or additionally, be provided by appropriate flip-chip assembly techniques. According to these techniques, two elements are electrically interconnected through flip-chip bumps attached to one of the elements. The flip-chip bumps may include solder bumps, gold balls, molded studs, or combinations thereof. The bumps may be formed or placed on a semiconductor die using known techniques such as evaporation, electroplating, printing, jetting, stud bumping, and direct placement. The die is flipped, and the bumps are aligned with corresponding contact pads of the other element. 
     By now it should be appreciated that there has been provided an improved packaged semiconductor sensor device and a method of forming the improved packaged semiconductor sensor device. Circuit details are not disclosed because knowledge thereof is not required for a complete understanding of the invention. 
     Although the invention has been described using relative terms such as “front,” “back,” “top,” “bottom,” “over,” “above,” “under” and the like in the description and in the claims, such terms are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the disclosure described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. 
     Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. Further, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. 
     Although the disclosure is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims. 
     It should be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps may be included in such methods, and certain steps may be omitted or combined, in methods consistent with various embodiments of the invention. 
     Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence. 
     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 
     The embodiments covered by the claims in this application are limited to embodiments that (1) are enabled by this specification and (2) correspond to statutory subject matter. Non enabled embodiments and embodiments that correspond to non statutory subject matter are explicitly disclaimed even if they fall within the scope of the claims.